Feature : merge layers

Added feature to merge a layer with the layer below
Done at #jsconfeu2014 :)

New icon in icons : merge-icon.png
TODO : reorder the icons folder, it's waaaaaaay too messy !!
Enhancement : Layers merge

Added drawing test + fixed drawing test runner on Mac OS X
increased the timeout for Travis tests
Merge pull request #220 from juliandescottes/enhancement-layers-options

Enhancement layers options