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
Merge pull request #88 from TobiasRaeder/add-cargo-features

Add cargo features for selecting a GTK version
Rename Gtrue and Gfalse to GTRUE and GFALSE

To comply with #[warn(non_uppercase_statics)]
Merge pull request #90 from TobiasRaeder/non-uppercase-statics

Rename Gtrue and Gfalse to GTRUE and GFALSE
Prepend TreeView getters with 'get_' for consistency
Merge pull request #93 from oakes/getters

Prepend TreeView getters with 'get_' for consistency
New widget : TreeSelection
Add function in relation with TreeSelection
Update to last rust compiler version