[Android] Added fexible and editable android manifest to config

[DanBrooker]

This is a follow up to my comments in #181 on the need to have a flexible way to build and edit the AndroidManifest.xml

This PR provides a manifest object stored as nested hashes. It allows for user to change all default manifest options and add additional nodes arbitrarily. It removes the hardcoded manifest xml that exists currently

For instance to set the theme on application in the Rakefile

app.manifest.child('application')['android:theme'] = '@style/AppTheme'

We can also grab the main activity and add a property to it

app.manifest.child('application').children('activity')[0]['android:label'] = 'New Label'

I've made an effort to not break the existing config vars such as services, permissions etc