Fix Android SoftKeyboard & DatePicker Plugin deprecations

Extend the newer CordovaPlugin class rather than the deprecated Plugin class (it appears this class is removed altogether in the Cordova 2.7.0 source).

I followed the guide at: 
http://docs.phonegap.com/en/2.6.0/guide_plugin-development_android_index.md.html#Developing%20a%20Plugin%20on%20Android

Tested with Cordova 2.6.0