Add initial cmake file for building on Linux

Only tested on gcc 4.8 so far
Remove useless message
Deleting abstract classes without virtual destructors is undefined
Add Data class to indirect all data loading through

Linux, for example, has case-sensitive filenames, so the plan is we can
add a case-insensative file loader here.
Remove excess 'data/' prefix for filenames in testPck
Data: If we fail to load a file, try a lower-case version

Means I don't have to play around with filenames in Linux as the CD
iso mounts with lowercase filenames by default
Merge remote-tracking branch 'pmprog/master' into linux
Merge pull request #3 from JonnyH/linux

Linux support & resource datafile loading abstraction