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