Postgres test floc 1042

Fixes https://clusterhq.atlassian.net/browse/FLOC-1042

If this is suitable, I will add the new dependencies (currently just the latest https://pypi.python.org/pypi/psycopg2) to `setup.py`. Each dependency must also go in the internal documentation for packages used.

This uses `psycopg2` which requires, on OS X for example ``brew install postgresql``. Perhaps it would be better to use http://python.projects.pgfoundry.org, a pure Python implementation. If not, should this just document that you need to install PostgreSQL?.
Problematic on my machine too:
```
[ERROR]
Traceback (most recent call last):
  File "/home/dave/flocker/flocker/acceptance/test_postgres.py", line 170, in create_database
    with connection_to_application.cursor() as application_cursor:
exceptions.AttributeError: __exit__

flocker.acceptance.test_postgres.PostgresTests.test_moving_postgres_data
```

I was just talking to Adam about this in the office, I think just use `cursor = connection_to_db.cursor()`