Rescue CD support & checking for defined media

Added new media type - 'Rescue' with some basic description and added
few checks so only defined ISOs will be displayed. To prevent
displaying Rescue CD for 12.2 and 12.3 till it gets done.
Live CDs are going to become Live DVDs
Make CD/DVD strings easier to translate
Merge pull request #24 from miska/master

Basic preparations for Rescue CD