user model for configuring multiple user accounts

Each user, identified by the `username`, will need to track their
own configurations: a `.trc` file for accessing their tweets and
a `.current-tweet` file for caching the latest tweet processed.

This also sets up where the config files will be stored and how
they are accessed.  All environment and test handling setup is
in here as well.
Merge pull request #8 from kellyredding/kr-users

user model for configuring multiple user accounts