Added alembic configuration, created a migration script

I have a couple questions on this:

1. Is there ever a time we will have multiple usernames and packages on a single message?
  - Can you provide me some examples of how to generate messages from packages that will have examples of this?  I'm currently only testing with fedmsg-logger

2. I can't default the columns using existing data, so the only time they get a default is through the add function.  If we started passing the needed data to _msg, we could default off of that.

3. Do we want to set any defaults in the migration script for existing logs or is it ok to have them nullable?


I still need to add the tests and documentation for this but wanted to discuss it a little first.