Url validation #15

Hello,

Here are some scenarios where is_url shows unexpected behaviour:

1. http://google.abcdefghi is valid but is_url states False in its test suite- constraining TLD to length 6 but TLD length specified by RFC 1034 is 63 octets. Real TLDs available here of length >6 http://data.iana.org/TLD/tlds-alpha-by-domain.txt
2. url with host ending in dot: http://www.foo.bar./ is valid as per https://www.w3.org/Addressing/URL/url-spec.txt#page13 while is_url returns False