Add a HTML module, including character escaping

[kud1ing]

A central module libstd/Html.rs could be useful. Golang has one: http://golang.org/pkg/html/

• HTML-escaping should probably implemented ontop of libcore/iter::map or something similar.
• Escape at least ", <, >, \, &

[kud1ing]

Closing this in favor of the Wiki page: https://github.com/mozilla/rust/wiki/Lib-html