Add vectorize decorator.

Analogous to NumPy's vectorize decorator.
Make _key_and_push check for distarrays.

In Context in client.py
Add local decorator to Context.

stolen from odin.py. However functionality is reduced. It's first
argument must be a distarray and no others can be DA's.
Shouldn't we make numpy available on the engines?
WIP vectorize changes
vectorize and local decorator tests.
WIP fix vectorize decorator in client.py