expressions cache hash
Merge branch 'master' into hash-cache

Conflicts:
	blaze/expr/reductions.py
numpy: count only uses notnull if floating dtype
Symbol -> symbol.  Cache symbols
expr: cache Expr.__getattr__
expr: test that symbol caches
expr: explicit symbol cache
expr: avoid generators in cheap situations
docs: fix doc test
symbol subs respects cache
h5py supports map
h5py uses thread_pool by default
symbol_key function supports dshape kwarg
expr: correct agg split shape when some dimensions don't match
Merge branch 'master' into h5py-thread-pool

Conflicts:
	blaze/compute/pyfunc.py