Unified config.xml syntax

This unifies the config.xml used by iOS and Android. Now <feature> tags can be used
to add plugins. The changes are non-intrusive so the older versions of the config.xml
can still be used after this change. The value of a param named "android-package"
determines the class for the plugin.

Resolves 
https://issues.apache.org/jira/browse/CB-1109
https://issues.apache.org/jira/browse/CB-1108
for Android