Execute PaperScript using new Function() rather than eval() and with() {}

This results in some impressive speeding improvements, as modern JS engines are finally able to optimize the resulting code.
Change the way PaperScripts are executed in Node.js

Requiring a PaperScript returns an initialize method which receives the Canvas argument.
Update to latest Prepro.js that exposes all globals.
Instead of creating the actual canvas we can also just provide a size.
Bail out of Item#transform() immediately if there is nothing to do.
Shorten code a bit.
Merge branch 'refs/heads/paperscript-refactoring'

Conflicts:
	src/core/PaperScope.js
Clean up code and comments.
Fix comments.
Merge git://github.com/paperjs/paper.js