Setup db-migrate and create migration to rename poets.name to username

db-migrate and pg must be installed globally with npm install -g
db-migrate should be run from the db directory, where the migrations
folder and database.json live.

database.json contains the configuration db-migrate uses to access the
database. You can set up multiple environments if desired, but it looks
for 'dev' or 'development' by default, so I'm just using that.
Rename poet.name to poet.username
Add info about migrations to the database README
Include db-migrate in package-json as devDependency
Merge pull request #61 from jimguys/name-to-username

Rename Poet name to username