Allow to specify license key via plugin parameter

As requested in #31 PGB currently does not allow customizing any steps of the build process, thus preventing the use of this plugin because there would be no way to customize or provide the Android license key.

This pull request changes the installation procedure of the plugin **requiring** to provide the license key upon installation (when installing using the cordova cli) and upon configuration in the config.xml file (when configuring for PGB). The changes are minimal although they lead to a different installation procedure which is not compatible anymore with the previous one, based on copying the res xml file to the platforms folder (I tried to figure out whether it was possible to keep a backwards compatible way to install the plugin but that doesn't seem to be possible).