Added legislator and legislator_session models

- legislator: holds data on legislators that should not change from
session to session

- legislator_session: holds data specific to legislators in a
particular session (e.g. state, district, role, party)
Added party to legislator_session model
South migration for new models (legislator and legislator_session)
Outline of code to load bioguides for Congresses
South migrations
Updated models

- Added session field
- Allow nulls/blanks for some fields that vary between
house and senate
Finished command to load legislator bios from XML bios
Uses NYT Congress API to fill in missing ICPSR Ids from Govtrack
SQL dump with legislator bios and ideology loaded
Migrate foreign keys
Vagrant config for VM
Re-arranged models to use ForeignKeys properly
Now uses foreign keys for legislators
Use NYT Congress API to find missing ICPSR Ids
Finds missing ICPSR Ids by using bioguide Ids
User V2 config style
Added lxml requirement (for parsing XML files)
Added polarization_over_time view/url

This returns JSON one entry per chamber per congress with a measure
of polarization (distance between the average ideology of each party)
Basic setup for VM - install postgres/python/other dependencies
Added vagrant config to gitignore