Merge pull request #86 from GuillaumeGomez/master

New trait and new widget
Development in progress
New Widget : IconView
Update to last rust compiler version
Merge pull request #87 from GuillaumeGomez/master

New widget : IconView
Add cargo features for selecting a GTK version

This should fix or at least help to solve #66
Use new semantics of multiple #[cfg(..)] attributes