Updated documentation

* Updated README.md
* Shortened and spread out `rgtk.rs` and `gtk/mod.rs` documentation
* Add documentation stubs for `gdk/mod.rs`, `glib/mod.rs` and `cairo/mod.rs`
Oh sweet, i had know idea, that was even there. Right now it is showing nothing though: rust-ci.org/jeremyletang/rgtk/doc/rgtk/