Merge pull request #6 from xguru/master

Plugin class is deprecated from phonegap 2.7
Update changelog

### MAY, 12, 2013 ###

* Upgraded Plugin class to CordovaPlugin, thanks [@xguru](https://github.com/xguru)
* Added trackSocial and trackCommerce, thanks [@xguru](https://github.com/xguru)
Update README.md