Update esrefactor to rev 8ff2b6238b (Apr 15 2013).

https://code.google.com/p/esprima/issues/detail?id=414
Benchmark corpus: update jQuery Mobile to its latest version.

http://code.google.com/p/esprima/issues/detail?id=354
Benchmark corpus: use YUI latest version.

http://code.google.com/p/esprima/issues/detail?id=354
Add contribution guide in the form of CONTRIBUTING.md.

https://code.google.com/p/esprima/issues/detail?id=419
Update esrefactor to revision 34bc6e461e (Apr 21 2013).

https://code.google.com/p/esprima/issues/detail?id=420
Implement the rename refactoring demo.

https://code.google.com/p/esprima/issues/detail?id=420
Update esrefactor to revision c5081d5385 (Apr 23 2013).

https://code.google.com/p/esprima/issues/detail?id=420