Only load the facebook SDK if it has not yet been loaded.
Update README.md

Updated with changes specified in #622
Update README.md

Updated README, fixed incorrect syntax for login https://github.com/Wizcorp/phonegap-facebook-plugin/issues/609
Merge pull request #633 from zipscene/master

Only load the facebook SDK if it has not yet been loaded
Merge pull request #636 from Wizcorp/develop

Merge develop