Upgrade Postgres.py to 2.1.0

I want to use __new__ in the AccountElsewhere model to vary the actual
class used based on the platform coming back from the query, and this
new version let's us do that.
Upgrade to postgres 2.1.1

Another fix to make __new__ easier to use
Implement subclassing of AccountElsewhere

This required some modifications to Postgres.py (released as version
2.1.1). Now we can have a subclass of AccountElsewhere for each platform
we support, and querying the elsewhere table will return an instance of
the appropriate subclass.