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
Merge branch 'master' into gh-pages