Added __init__.py to make nengo_mpi a package.
Started a test dir working with py.test.

Based largely on ocl's version of the same.
Removed test_examples.

Haven't been able to find a way to run these on the GPC with nengo_mpi.
Has to call __import__ to get a hold of nengo's test, but this causes
it to import GPC's 0.11 version of ipython.
Fixed bugs relating to probes.

* Simulator n_steps wasn't getting updated each step.

* Using unsigned long long ints as the key type to avoid overflow.
Fixed bug with PyFunc operators.

Previously couldn't handle the case where a 0-d array was
returned by a function.  Kind of a hacky fix, replace all
functions with functions that first check whether the output is
an ndarray, and if so, simply add 0.0 (which, in the case of
0-d arrays, returns a float). I kind of hate 0-d arrays.