Docs for IS_URL

    Rejects a URL string if any of the following is true:

       * The string is empty or None
       * The string uses characters that are not allowed in a URL
       * The string breaks any of the HTTP syntactic rules
       * The URL scheme specified (if one is specified) is not 'http' or 'https'
       * The top-level domain (if a host name is specified) does not exist

    (These rules are based on RFC 2616: http://www.faqs.org/rfcs/rfc2616.html)

    This function only checks the URL's syntax. It does not check that the URL
    points to a real document, for example, or that it otherwise makes sense
    semantically. This function does automatically prepend 'http://' in front
    of a URL in the case of an abbreviated URL (e.g. 'google.ca').

    If the parameter mode='generic' is used, then this function's behavior
    changes. It then rejects a URL string if any of the following is true:

       * The string is empty or None
       * The string uses characters that are not allowed in a URL
       * The URL scheme specified (if one is specified) is not valid

    (These rules are based on RFC 2396: http://www.faqs.org/rfcs/rfc2396.html)

    The list of allowed schemes is customizable with the allowed_schemes
    parameter. If you exclude None from the list, then abbreviated URLs
    (lacking a scheme such as 'http') will be rejected.

    The default prepended scheme is customizable with the prepend_scheme
    parameter. If you set prepend_scheme to None then prepending will be
    disabled. URLs that require prepending to parse will still be accepted,
    but the return value will not be modified.

    IS_URL is compatible with the Internationalized Domain Name (IDN) standard
    specified in RFC 3490 (http://tools.ietf.org/html/rfc3490). As a result,
    URLs can be regular strings or unicode strings.
    If the URL's domain component (e.g. google.ca) contains non-US-ASCII
    letters, then the domain will be converted into Punycode (defined in
    RFC 3492, http://tools.ietf.org/html/rfc3492). IS_URL goes a bit beyond
    the standards, and allows non-US-ASCII characters to be present in the path
    and query components of the URL as well. These non-US-ASCII characters will
    be escaped using the standard '%20' type syntax. e.g. the unicode
    character with hex code 0x4e86 will become '%4e%86'

    Args:
        error_message: a string, the error message to give the end user
            if the URL does not validate
        allowed_schemes: a list containing strings or None. Each element
            is a scheme the inputed URL is allowed to use
        prepend_scheme: a string, this scheme is prepended if it's
            necessary to make the URL valid

    Code Examples::

        INPUT(_type='text', _name='name', requires=IS_URL())
        >>> IS_URL()('abc.com')
        ('http://abc.com', None)

        INPUT(_type='text', _name='name', requires=IS_URL(mode='generic'))
        >>> IS_URL(mode='generic')('abc.com')
        ('abc.com', None)

        INPUT(_type='text', _name='name',
            requires=IS_URL(allowed_schemes=['https'], prepend_scheme='https'))
        >>> IS_URL(allowed_schemes=['https'], prepend_scheme='https')('https://abc.com')
        ('https://abc.com', None)

        INPUT(_type='text', _name='name',
            requires=IS_URL(prepend_scheme='https'))
        >>> IS_URL(prepend_scheme='https')('abc.com')
        ('https://abc.com', None)

        INPUT(_type='text', _name='name',
            requires=IS_URL(mode='generic', allowed_schemes=['ftps', 'https'],
                prepend_scheme='https'))
        >>> IS_URL(mode='generic', allowed_schemes=['ftps', 'https'], prepend_scheme='https')('https://abc.com')
        ('https://abc.com', None)
        >>> IS_URL(mode='generic', allowed_schemes=['ftps', 'https', None], prepend_scheme='https')('abc.com')
        ('abc.com', None)

    @author: Jonathan Benn