Vectorizable pow, exp, and log.
Merge branch 'master' of github.com:halide/Halide
Squashed HL_NUMTHREADS warning now that the default is sane.
You can finally cast Images to Arguments.

Previously you had to go via buffers, which was annoying.
log, exp, and pow handle constants more gracefully
fast_pow now casts non-floats to floats
Better handling of integer powers, and a test for it.
Merge branch 'master' into nick-sandbox