Implementing duplicate packages

Replaces https://github.com/joomla-projects/com_localise/pull/167

See #165

This will let use master packages to create a specific language package where the files to include are already highlighted.
![screen shot 2014-07-06 at 07 37 04](https://cloud.githubusercontent.com/assets/869724/3488440/79545ec4-04e3-11e4-87b8-7213d7b8c5dd.png)