Enable test_cmdcallback.py

Due to an error in the names of the two classes this test module was being skipped altogether. Without this PR:
```
jrs-macbookpro-25031:OmeroPy cblackburn$ ./setup.py test -s test/integration/test_cmdcallback.py 
[...]
=========================================================================== test session starts ===========================================================================
platform darwin -- Python 2.7.4 -- pytest-2.5.0
collected 0 items / 1 skipped 

======================================================================== 1 skipped in 2.67 seconds ========================================================================

```
With this PR the tests should run, and hopefully, pass:
```
jrs-macbookpro-25031:OmeroPy cblackburn$ ./setup.py test -s test/integration/test_cmdcallback.py 
[...]
=========================================================================== test session starts ===========================================================================
platform darwin -- Python 2.7.4 -- pytest-2.5.0
collected 6 items 

test/integration/test_cmdcallback.py ......

======================================================================== 6 passed in 6.41 seconds =========================================================================
```
--rebased-to #1911