repr on arrays doesn't crash
wip: repr behaves differently for different datashapes
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