Added sdl2 render backend
Fixed up sdl1 backend to work with new changes, made sdl version a cmake option
moved code common to both sdl versions out into a seperate file
use sdl1 with travis because ubuntu doesn't have sdl2 yet
use proper enums + atomic variable for render synchronisation
forgot to actually initialise mPath, so nothing was being removed from cache
use ppa for boost for travis
The version in 12.04 is too old
Merge pull request #60 from wheybags/sdl2

Sdl2 backend