Sessions support

This pull request adds the basic framework for using sessions, laying the groundwork for #86.

To get it working, do the following:

(Steps created by following the steps here:
https://github.com/jebas/connect-pg )

sudo apt-get install pgtap
npm install
db-migrate up

To change nodepg's password: (for the prod environment)
alter user nodepg with password 'wearetapvote';

Recommend using environmental vars TAPVOTE_DATABASE_SESSION_USER and TAPVOTE_DATABASE_SESSION_PASSWORD to use the nodepg account, which has minimal permissions.