Switch back to instanceof checks.

As people are started to subclass Paper.js classes...
Do not inherit selection behavior from Group in Layer.
Make font properties disappear in JSON.
Fix issues with deserializing selection states.
Support Size and Point arguments in Rectangle#expand()
Make JSON serialization work for Shapes.
Implement cached getInternalRoughBounds as well.
Fix error in #getGlobalMatrix()
Merge git://github.com/paperjs/paper.js