Add Empire campaign by Jendraz

There is now a clear public license statement for this campaign, so it's ready for merge: http://forums.wolfire.com/viewtopic.php?f=7&t=3797#p52613

I used a permalink from the Wayback Machine for it in the commit log and documentation.

All levels seem to load fine, so hopefully I did not mess files up when fixing hardcoded path - at any rate I did it in a separate commit, so the first commit contains the unmodified mod files.

The only path I haven't dehardcoded yet is :Data:Textures:blackfur.png, which should be made to point to Textures/Fur2.jpg (it's the same texture, just different name and format), but that triggers funky effects on the main character, so it needs more investigation.

Part of #31.