Adding 'core' plugin support.

I'd like, ultimately, to see upgrade-able core features moved to the plugin system. Specifically at this point, I'm thinking Sage.jar, SageTV7 STV, and Lucene.jar. Some tasks I think that are required to do this:

1. Add a 'CORE' tag to a plugin manifest. Essentially prevents removal of the plugin.
2. Implement a pre-launch plugin loader that would ensure that the CORE plugins are loaded and in the proper folders. This would be required especially if folders get moved around per issue #298.

I believe the local repository features @stuckless added a bit ago would enable the installers to contain the .zipped up plugin source files in the read-only app folder, which would provide a source for the pre-launch plugin loader to access if necessary on first run, which would then get the right files in the right place for the specific install.