Extract legislator methods to mixin module

Person class cannot be assumed to be legislator in
case when dealing with executive branch officials.

Extracting module allows for better handling of this
functionality.

#24
Add StateLegislator subclass of Person

This change requires that the MongoDB legislators
collection migrate to people collection. The steps
necessary for an existing db are as follows (in mongo shell,
from command line):

db.legislators.find().forEach( function(doc) { if (doc._type === 'person') { doc._type = 'StateLegislator' ;  db.people.insert(doc) } } );
db.legislators.find().forEach( function(doc) { if (doc._type === 'FederalLegislator') { doc._type = 'StateLegislator' ;  db.people.insert(doc) } } );
db.legislators.drop()

For new development, the README has been updated accordingly.

#24
Refactor Person class methods to service objects

Building block to allow per instance setting of source API, etc.
Move councilmembers to own subclass of Person

Some placeholder code for determining whether to return which person
type. Will get cleaned up as we expand to executive classes.

Also includes loading of seed city data from hardcoded files. Expected
to be replaced.
Fix issue w/ type params for identifier action