OMERO.web WSGI additions

Adds a `wsgi.py` so that we can use other WSGI web servers with OMERO.web. @glencoesoftware in particular been using Gunicorn (http://gunicorn.org/) in recent months to perform certain types of deployments.

Example use with Gunicorn:

```
pip install gunicorn
cd dist/
bin/omero config set omero.web.debug True
cd lib/python/
gunicorn -w 5 omeroweb.wsgi:application
...
```

This will start a fully functional Gunicorn instance with static file handling (`omero.web.debug=True`) and five workers. This is largely identical to the existing FastCGI + flup way we all regularly deploy OMERO.web in production.