Fixed #20003 -- Improved and extended URLValidator

Another attempt at fixing issue 20003, as it was previously attempted in #998.

This adds support for user:password in URLValidator supported URLs.

Additionally, the test suite has been improved by adding test URLs from
http://mathiasbynens.be/demo/url-regex (with a few adjustments, like
allowing local and reserved IPs).

The previous URL validation RegEx failed this test suite on 13
occasions, so I also updated the URLValidator based on
https://gist.github.com/dperini/729294 to correctly validate all URLs in
the test suite. Among other improvements, this also adds support for
IPv4 mapped IPv6 addresses like `::ffff:192.0.2.128` and support for
unicode domain names.

Docs have also been improved.
It makes the lines 78-80 in the regex very long though. The current formatting increases the readability of that regex because each line of code handles exactly one part of the hostname/domain/tld.

If you still think that it's worth it I can break up these lines into multiple lines and insert the `unicode_letters` variable name.

I also tried string formatting using the `.format()` method, but that causes collisions with the curly braces in regular expressions.