Zoom initial implementation. No UI, only bound to mousewheel. Everything is broken, to amend !
Added utilities for alpha-composition in FrameUtils, for future usage ... maybe
feature : zoom

- Created AbstractRenderer in rendering package
- Created CachedRenderer and CachedFrameRenderer to extract basic frame
  caching logic from DrawingController
- Created RendererManager to synchronize updates made to several Renderer
  settings
- Moved FrameRenderer from pskl.rendering to pskl.rendering.frame
- Fixed the resize of the drawing area when the window is resized
feature : zoom : continued

- simplified Renderer(s) architecture (removed decorator,
  CachedFrameRenderer simply inherits from FrameRenderer now)
- keeping AbstractRenderer to act as interface
- fixed issue with layers : forgot to clone the first frame while merging
  and therefore was modifying the original frame when I just wanted to
  create a tmp frame (FrameUtils.js)
- extracted the mousemove throttling delay used in DrawingController to
  Constants.js and reduced it from 40ms to 10ms
feature : zoom :

- Extracted layers rendering logic from DrawingController to dedicated
  class
- Turned RendererManager into Composite renderer (extends
  AbstractRenderer)
- AbstractRenderer no longer contains a render(frame) method, implementing
  the render differs too much between my current renderers to impose a
  single signature, but I should improve this later if too much time on ma
  hands
Forgot 2 files !
feature : zoom

- Added MinimapController that displays a frame on the animated preview
  when zoomed in
- Added bounds for the offset to make sure it doesn't go crazy
- Added new utility Math.js with a minmax function
- TODO : the minimap controller has a lot of dependencies, see if could be
  cleaned up
- TODO : DrawingController knows the size of the picture it has to render
  only indirectly, which makes it hard in some cases (such as boundary
  checking performed during setOffset)
fix : LocalStorageService deserialization
merged from master
fix : renderer resize on window resize
feature : zoom-level

- Minimap usability : mouseup and mousemove events are now plugged on
  document.body instead of the minimap's controller. This way the user can
  move his mouse outside the container to keep moving the map's frame.
  Also the mouseup information is no longer lost if it occurs outside for
  the minimap.
feature : zoom-level

- Fixed bug with layer rendering when moving drawing offset (bad
  redraw-flag checking)
feature : zoom-level

- Fixed bug with layer rendering when moving drawing offset (bad
  redraw-flag checking)
feature : zoom level

- fix : removed duplicated code between ImageResizer and CanvasUtils
  (disabledImageSmoothing utility method)
- added pskl.utils.UserAgent, basic user agent sniffer. I need it to sniff
  out IE10 for frame rendering (and it's not possible to feature detect
  here). Can check isChrome, isFirefox, isIE and get the version for each
  of them
- added resizeNearestNeighbour in ImageResizer. Readapted from piskel
  website, this allows us to 1 - resize without AA on IE10, and 2 - add a
  pixel gap to reenable the GRID
- finally : added back support for GRID !
- also extracted the 'zoomed out background color' as a constant in
  Constant.js
Merge branch 'master' into zoom-level
renamed all references to dpi to zoom for consistency (also wtf dpi anyway !)
feature : zoom level : code review

+ added explanatory comment for CanvasUtils.disableImageSmoothing
+ detect browser for chosing wheel/mousewheel event in DrawingController
+ create ABSTRACT_FUNCTION constant to be reused forabstract methods
feature : zoom level : code review

+ Switched from BITWISE OR 0 (x | 0) to Math.floor for readability
Merge pull request #141 from juliandescottes/zoom-level

Zoom level
Merge with master after PR zoom-level