Feature : Save Piskel project as File

First commit :
Removed Local storage feature
Added 'download project' 'open project' options

First attempt at simplifying right panel.

To be continued ...
Merge branch 'master' into feature-export-to-file
temp commit
Merged changes
Merge branch 'master' into feature-export-to-file
cleanup of import tab
Image dnd first implementation
Merge branch 'master' into feature-export-to-file
Merge branch 'feature-dnd-images' into feature-export-to-file
Create 2 new dialog controllers

Image import is now triggering a popup after selecting the file.
Same for local saves.

Drag and drop of .piskel files opens the piskel immediately !

Remains to do :
- redesign the dialog for import image and browse local
- create dialog for recover session
- improve recover session to handle more than the last session
Fixed Gruntfile for ghpages export
cleanup
cleanup
Fixed mime type bug on Firefox
Fix gif worker URL on IE11 appengine mode
Switch back to imgstore-a
Cleanup + updated tooltips
Capture events on tmp download link created by FileUtils
fixed lighten reset + fixed history bug
restored history snapshot interval to 50