Split widget trait
- remove Wrap / get_widget methods from Widget trait. these don't need to be public.
- move the previous methods to the traits FFIWidget, the user cannot see it.
- Widget need FFIWidget to be implemented

This ensure that a user cannot create a Widget other than one defined in rgtk as the trait FFIWidget is not public.
fix travis
Merge pull request #39 from jeremyletang/travis

fix travis
New functions in widget
New functions in Widget
Changed to List<Box>
Merge pull request #41 from GuillaumeGomez/master

New functions in widget
try to enable rust-ci doc
try fix doc upload
Add enums in gdk
Add new widget functions
Merge pull request #42 from GuillaumeGomez/master

Add enums in gdk
fix travis
Merge pull request #43 from jeremyletang/travis

fix travis
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