Change cfg(GTK_3_x) to cfg(feature = "GTK_3_x") to make cargo build work

Master branch doesn't let me specify rgtk as dependency in Cargo.toml
because features are not matching.
Master checks with cfg for GTK_3_12 but it should be feature="GTK_3_12".
add author
Merge pull request #99 from buster/cargo-features-fix

Cargo features fix
Change .travis.yml to use cargo
make Travis use GTK3.6
remove Makefile targets that don't build glue

Since examples, docs, rgtk can be build with Cargo,
the makefile may not contain them, only the glue
Let examples be build with Cargo

Needs source to be moved to examples/src/
build examples on Travis
revert Travis build of examples due to travis only having 3.6
move examples to seperate directories
let travis build the examples, except gtktest
Fix GTK Version in .travis.yml globally
fix typo in gtktest Cargo.toml.. should fix Travis build now
Merge pull request #102 from buster/make-to-cargo-travix-fix

Change .travis.yml to use cargo
fix warning IconSize in button.rs for version inferior to 3.10
Merge remote-tracking branch 'upstream/master'

Conflicts:
	examples/Cargo.toml