Added Gitter badge
Merge pull request #43 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md
Fix warnings
Cocoa opens a window with OpenGL context inside
Refactored window initialisation code for cocoa, making it more memory safe
Added events handling for left/right mouse clicking
Merge branch 'cocoa-window-support' of github.com:DavidPartouche/gl-init-rs into cocoa-window-support