Use NengoTestLoader to run individual tests

Now possible to run individual tests with
```python nengo_ocl/test/test_sim_npy2.py <module>.<class>.<function>```
where class and function are optional.

E.g.,
```
>>> python nengo_ocl/test/test_sim_npy2.py
EEEEEEEEE...EE
...
----------------------------------------------------------------------
Ran 14 tests in 3.891s

FAILED (errors=11)

>>> python nengo_ocl/test/test_sim_npy2.py test_simulator
EE
...
----------------------------------------------------------------------
Ran 2 tests in 0.002s

FAILED (errors=2)
>>> python nengo_ocl/test/test_sim_npy2.py test_simulator.TestSimulator.test_simple_direct_mode
E
...
----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)
```