Add chamber to Office model, allow 'US' for state

We need this to allow importing records from CSV in the
``load_metadata.run`` task.

Serdar suggested using 'US' as the state key for national-only
offices such as President.
Import office and party fixtures using MongoEngine

The ``mongoimport`` command that we used for the heavy lifting
didn't properly preserve district numbers as strings.

Closes #68