Update PushPlugin to be compatible with cordova-cli 3.0.0

Includes the following changes:

- plugin.xml updated the Cordova 3.0 format (based of off core Cordova plugins as a reference).  This is most likely incompatible with Cordova <3.0, so I've also removed the Cordova <2.3 special case.
- Updated PushPlugin.java Cordova imports (solution compliments of @mwager) .
- Update iOS files to support ARC builds, which is the default configuration via cordova-cli 3.0.0.