Change host to use ip address types provided by std.

Removes the custom IPv6Addr type and replaces it with the std one.
Parses IPv4Addrs to the std type using the parser described in the
url.spec.whatwg.org handling all edge cases. Add tests.

fixes #116

This is a breaking change. Version bumped to v0.5.0.

Author: pyfisch

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "url"
-version = "0.4.0"
+version = "0.5.0"
 authors = [ "Simon Sapin <simon.sapin@exyr.org>" ]
 
 description = "URL library for Rust, based on the WHATWG URL Standard"