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`
Add OSX shell instructions
Rename _Box -> Box using no_implicit_prelude, better docs
Rename some trait reexports
Added rust-ci doc link
Merge pull request #38 from mathijshenquet/better-documentation

Updated documentation