updated changelog !
Removed the obsolete 'me'sys.cacheImage' setting as it was not brining any real benefits
[#271] renamed `me.Viewport`  `screenToWorld` and  `worldToScreen` respectively to `localToWorld` and `worldToLocal`

Also improved documentation, overall this should avoid any
misundersanding/confusion on the function purpose
fixed a jslint issue
Quick fix to solve a flickering/blury display issue when lowering the FPS rate on cocoonJS

Better fix to be done in the future through ticket #99
Moved the timer code into a dedicated timer.js file for better code readability, and moved it under system
[#253] refactored how we managed the `orientationchange` event

Also exposed one new `device.orientation` property and one new
`ON_ORIENTATION_CHANGE" event channel (minpubsub)
[doc] fixed the readonly attributes for `me.device` properties
More refactoring around `me.device`