Changes to the database schema

  - Tweets are accessed via the coordinates of their squares
  - Store the number of rows and columns in each big square in the coords table
  - Create tables if they don't exist (for now)
Modify the TweetManager to support multiple users

TweetManagers must be given a session id when initialized.  They use that
session id to look up the details about their query in the database.

The tests most likely don't mean anything anymore and need to be fixed.
Add keyword groups to the tweet store

Can now specify whether the tweet was found in the first, second or
intersection group.
GeoPartitioner has been removed
Implement the display action

Opactiy and Venn diagram computation copied over from GeoPartitioner and old
Gathering.