Added debug task that runs 'ant debug'

I have added a task which uses the 'ant debug' command which is run using 'grunt phonegap:debug'. 

This generates an APK asset similar to the one from 'grunt phonegap:release' but it is not signed. 

By browsing to this APK asset from test hardware device, we can quickly install the APK output from our build. Then we use chrome://inspect to inspect the network traffic as an example - this is not possible using the signed 'ant release' APK.