Deal with ambiguous Geocoding for other States

Proposed change to deal with ambiguous geocodings on a state-by-state basis. This change does 2 things:

1. Adds a new setting (STATE_FILTER) in settings.py that takes as a value a two letter state abbreviation (e.g. PA)
2. `cicero_search.py` uses settings.STATE_FILTER to filter out ambigous addresses outside a given state.

Filter addresses on a state-by-state basis seems necessary since when
ambiguous addresses like '1234 Market St.' are entered the results may include
cities in states that the site does not serve.