Added some texture atlas features.

 - added name property to frame
 - added function to find a frame by its name.
I only added frame name support for the sparrow importer (don't know
the other very well).
Added some texture atlas features.

Fixed to previous incomplete commit.
Merge with upstream repo
Update to remote repo
Added basic support for using Spritesheet as a Texture Atlas.

 - Added 'name' property to a spritesheet frame.
 - Added a findFrameByName() method on SpriteSheet.
 - Implemented parsing of frame names for the Sparrow importer.
Merge pull request #5 from xastor/master

Support for using a Spritesheet as an image atlas.