add libhtml

Escape will escape the 5 chars <, >, &, ', and ".
Unescape will unescape a lot more characters.

- fn html::escape(&str) -> ~str
- fn html::unescape(&str) -> ~str
- struct html::fmt::Escape(&str)
- struct html::fmt::Unescape(&str)

References:
http://golang.org/pkg/html/
https://docs.python.org/3/library/html.html

The [wiki](https://github.com/mozilla/rust/wiki/Lib-html) claims this is desired.