First pass at merging the pack face kernels.  Not fully implemented for domain wall and staggered, and wilson remains buggy.
Fixed and optimized single-kernel packing method for Wilson.
Added callback experiment to face buffer, but leads to suboptimal performance, so is disabled.
Added staggerd support for merged face packing kernel.
All staggered dslash loads go through texture on Kepler.
The Ls parameter can now be set from the command line for dslash_test and invert_test.
Added domain wall support for merged pack kernels.
Domain wall: moved Ls dimension into y thread dimension to increase locality of gauge field, allowing for better caching.
Merge branch 'master' of github.com:lattice/quda.  Added merged-pack kernel support for non-degenerate twisted mass dslash also.