repr on arrays doesn't crash
wip: repr behaves differently for different datashapes
make top_to_bottom recursion look less scary
clean up top_to_bottom compute_down section
Remove Union
call pre_compute on any type change
h5py: pre/post_compute use .value if array is small
Merge branch 'repr-arrays-robust' of github.com:mrocklin/blaze into h5py-precompute
post_compute doesn't dispatch on scope

Instead it's a kwarg

Conflicts:
	blaze/compute/core.py
h5py: pre_compute doesn't collapse slice expressions
clean pre_compute on type change code
docstrings
provide optimize function as kwarg to compute chain
python: ensure that python pre_compute doesn't burn data
optimize on new data
add pre_compute as kwarg to compute chain
h5py: optimize to bubble slices down
h5py: slicing test on h5py dataset
h5py: optimize down binops
expr: small optimizations on subs