Merge pull request #13 from jocelynlih/ankur-refactoring

Moving Extensions into its own files our of Game Scene and Adding accessor types to Functions
Sketch rendering is now done on a per-sprite level with child sprites that contain sketches. They need to be animated, which will be next. Wanted to get this stuff in so I can sync with master.
Merged with master
A bit of code cleanup - moved the sketch rendering code to its own class.
Sketch animations are in
Added a comment to remind me about the necessity of caching sketch sprites.
Merge pull request #17 from jocelynlih/paul

Paul
Moving extension code into the extension file
Merge branch 'master' of https://github.com/jocelynlih/SwiftGameBook into ankur-refactoring