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 "ios-package"
determines the class for the plugin. Also changes the config.xml with the unified one
on the project template

Resolves 
https://issues.apache.org/jira/browse/CB-1109
https://issues.apache.org/jira/browse/CB-1108 
for iOS