Upgrade iOS Version of Plugin to Cordova 2.7.0

Change method signatures to match standard for Cordova 2.7.0.

Followed [plugin upgrade guide](https://github.com/phonegap/phonegap/blob/master/lib/ios/guides/Cordova%20Plugin%20Upgrade%20Guide.md#upgrading-older-cordova-plugins-to-270) on github.