Contraction code finally ready!

- Simple contraction kernels and gamma5 multiplication in contract.cu
	+ Includes time-slice contractions, plain volume contractions and contractions with a gamma5 inserted in the middle
	+ The output are the 16 dirac components of the contractions

- Covariant derivative implemented

So far only kernels are included, the interfaces will come in another commit
Merge branch 'quda-0.7' of ssh://github.com/lattice/quda into quda-0.7