Prepare release 0.5.2

Prepare 0.5.1 release

Merge branch 'master' of https://github.com/juliandescottes/piskel

Workaround for Chrome 41 issue Canvas + Radeon :

Issue opened at https://code.google.com/p/chromium/issues/detail?id=469906
Workaround in FrameRenderer.js is to decrease the width+height of the
fillRect of 1 pixel.
Issue seems to impact only Radeon users (not sure if all cards are
impacted)

Merge branch 'master' of https://github.com/juliandescottes/piskel

Update README.md

Prepare release 0.4.0

preparing release v0.3.0

Small cleanup zip export screen

Merge pull request piskelapp#208 from juliandescottes/feature-export-…

…to-file

Feature export to file