Add missing Canvas2DRenderingContext.restore() call

When this code is used in QtWebKit (and possibly other implementations), calling ctx.save() without a corresponding ctx.restore() call results in a resource leak internally.
Add retina support
Be more explicit in opacity calculation

Firefox currently throws several hundred errors from the _draw method:

`Expected a number but found '2.22045e-16'. @ from DOM`
Accommodate for my @5x DPI and your bitch ass @2x

Jk, but not all DPIs are 2x.
`.stroke` doesnt take arguments

Fixes chrome 35 error.
Fix parentheses for original math operation dfa4b51ad17876f935a735dcf3d74f3f66ef26ef

Also truncate decimal places for the FF errors
Add separate start/stop buttons to example page
Merge pull request #5 from okcoker/master

Few enhancements
Merge pull request #2 from robertknight/patch-1

Add missing Canvas2DRenderingContext.restore() call
requestFrame etc
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