Linux support & resource datafile loading abstraction

Add really-simple CMake build file for Linux (Only tested with gcc 4.7 & gcc 4.8)
Requires allegro 5.0 to be installed (Uses pkgconfig to find libraries etc.)
Add 'Data' class to simplify filesystem case differences (It prepends 'data/' then tries the supplied name, then a lower-case version of the same name before failing). This allows us to continue using UPPER_CASE (relative to the data directory) for loading resources for the time being, while leaving the door open for resource substitution in the future (for mods etc.)