TilemapLayer / Tileset performance and documentation

TilemapLayer
 - Added "delta scroll rendering"
   - Enabled (by default) via `renderSettings.enableScrollDelta`
   - Dramatic CPU reduction: 30% to 2% scrolling 50x37 region
   - Side-effect: fixed some Chrome de-opt issues with `render`
   - Various rendering updates and removal of many `_mc` field
 - Made `index` a read-only propert that reflects the layer.index
 - Creates a local cache from tile index -> tileset, added `resetTilesetCache`
   - Also prevents crash when an index not associated with a tileset is
     used; see `missingImageOverfill`.
 - Various debug options moved to `debugSettings` and simplified; documentation updated.
   - Uses `debugSettings.missingImageOverfill` and `tileColor` (deprecated) to display invalid tileset images
 - Only change the context alpha when the tile alpha changes; in Chrome 38
   this removed a .5% penalty
 - Added `renderSettings` which controls misc. rendering/optimizations.
 - Updated documentation to @member, clarified

Tileset
 - Added `containsTileIndex` method to allow being queried if the specific tile index is handled/supported.
 - Uses a firstgid bias and interlacing to fit the data in single densely packed array.
 - Updated documentation to @member, clarified
Merge branch 'dev' of https://github.com/photonstorm/phaser into wip-tilelayer
TilemapLayer / Tileset

- Minor regression fix, increasing argument specifity/documentation
Merged in 'wip-tilelayer'

  - This brings over the canvas edge-delta support
  - Also added really cheap "prompt" to switch between renderers.. you'll know it when you see it.