Moving omeroweb unittests to pytest

This is the same as #1460 "rebased" onto develop by cherry-picking the commits. The three test targets all run to completion though one has a warning thrown (exceptions omitted from output). The key here is that the tests run okay and use pytest. Any problems with the tests themselves should be addressed in a separate PR.

```
jrs-macbookpro-25031:ome cblackburn$ dist/bin/omero web test --testpath webclient
================================================================================================================================== test session starts ===================================================================================================================================
platform darwin -- Python 2.7.4 -- pytest-2.3.5
collected 1 items 

webclient/tests/test_webclient.py WARNING:omero.gateway:ApiUsageException on <class 'omero.gateway.OmeroGatewaySafeCallWrapper'> to <d4efa656-c9bc-4ea5-8421-4775d21831bcomero.api.IAdmin> lookupExperimenter(('weblitz_test_user',), {})
```
...
```
.**

================================================================================================================================ 1 passed in 4.97 seconds ================================================================================================================================
jrs-macbookpro-25031:ome cblackburn$ dist/bin/omero web test --testpath webadmin
================================================================================================================================== test session starts ===================================================================================================================================
platform darwin -- Python 2.7.4 -- pytest-2.3.5
collected 18 items 

webadmin/tests/test_webadmin.py ........x.........

========================================================================================================================= 17 passed, 1 xfailed in 17.56 seconds ==========================================================================================================================
jrs-macbookpro-25031:ome cblackburn$ dist/bin/omero web test --testpath webgateway
================================================================================================================================== test session starts ===================================================================================================================================
platform darwin -- Python 2.7.4 -- pytest-2.3.5
collected 17 items 

webgateway/tests/test_webgateway.py ..............I<./../../../bin/omero -s localhost -k 7550c78c-d656-4777-9568-9156ca4be7e8 -p 4064 import -d 6 -n weblitz_test_priv_image /Users/cblackburn/Work/repos/ome/dist/lib/python/omero/gateway/scripts/imgs/CHOBI_d3d.dv
...**

============================================================================================================================== 17 passed in 120.76 seconds ===============================================================================================================================
```