Added Phonegap support

- Updated angular-facebook.js
Now, you can use the setLoadSDK method in the provider to stop automatic
load of Facebook SDK script. This is helpful for Phonegap projecs which
need to delay the loading up to deviceready event is being triggered by
Phonegap API.
Optionally, you can use the simpler way by passing false as second
argument to the init method from the provider.

e.g. FacebookProvider.init(initOptions, false);

- Added angular-facebook-phonegap.js
This script should be used in Phonegap projects. It delays the Facebook
SDK loading up to deviceready event being triggered by the Phonegap API.

For this to work you need first to disable automatic loading of SDK and
second, to specify the setting 'localSDK', with the source path where
the Facebook SDK file should be loaded.
This can be done using the setInitCustomOption method from the provider.

e.g. FacebookProvider.setInitCustomOption('localSDK',
'my_local_facebook_sdk_file.js');
// format '.js' can be omitted if desired