Merge pull request #3 from smittal/master

Another 60% speedup; now runs smoothly on my Moto X.
Basic working pencil tool
Add pencil png
Better buttons.
Selected state for pencil button.
Multicolored pencil.
Change cache life to only 30 minutes.
Hide the pencil button from mobile users because it's horribly broken.
Use right ID for pencil button.
Update README
Update README.md
Add lots of comments and fix most jslint errors