Factor out LLVM backend

The LLVM backend is now available as a seperate project on github.
Adapt travis testing
Only build llvm backend if testing llvm
Make sure we use the official version of idris-llvm for testing
Introduce class Negatable
Rename the Negatable class to Neg
Make unary "-" use the Neg typeclass
Add support for the primitive prim__negFloat : Float -> Float
Rename float negation primitive for more uniformity
Add test for new float primitive prim__negFloat
Merge branch 'master' of github.com:markuspf/Idris-dev into markuspf-master

Conflicts:
	libs/prelude/Prelude/Classes.idr
	src/Idris/ParseOps.hs