implements clipping masks using canvas blending
style guide cosmetics
don't set this.ctx in _enter/_exitClip
fixes broken transformation in masks
remove forgotten console.logs
some light refactoring

- renamed _enterClip/_exitClip to clipStart/clipEnd
- moved those methods to end of RendererVisitor
refactors _mask functionality to use clipStart/clipEnd
inline comments explaining clipping mask functionality
introduce DisplayObject._isContainer property

simplifies the rather tedious (and, in renderer.js, repetitive) task of checking whether or not a DisplayObject is a container (DisplayObjectContainer or SimpleButton)
fixes mushroomer regression

BitmapData::draw was not behaving correctly when the drawn DisplayObject contained masks
Merge pull request #844 from claus/clipmask

Makes text as clipping masks work