Initial integration with node-db-migrate.

This converts the CREATE TABLE code in lib/mysql.js to a migration
that can be rolled-back. Also added a test to ensure that it works.

Migrations are automatically run when the app is launched and when
the test database is set up at the beginning of each test. The latter
could get slow, but there are ways around it when that happens, such as
getting a mysqldump of the fully migrated state and executing it as a
series of SQL statements. But we'll cross that bridge if/when we come to it.
Added bin/db-migrate, which is a modified version of the db-migrate script.

I'm not a huge fan of this approach. Not only are we copy-pasting
code from db-migrate, thereby violating DRY, but the code uses undocumented
APIs which could break at any time. A better approach would be to
issue a pull request to node-db-migrate (or just use a fork) that
refactors it in a way that allows us to do what we need without
having to copy-paste the code. But figuring out how to do the refactoring
will take time, which we don't have much of right now.

Doing this also meant not calling createTables() in lib/mysql.js, i.e.
users need to run bin/db-migrate manually to ensure migration race
conditions don't occur between multiple server processes that are started
simultaneously.

I still need to:

* Document bin/db-migrate in the README, adding instructions on how to
  migrate up/down and create migrations.
* (optional) Add some code in app.js that checks to see whether we're
  at the latest migration, and report an error if not. This will be
  more helpful to developers than running code that expects a newer db
  schema and exploding confusingly.
added 'start' target to makefile which does migrations, then runs up.
added section on Database Migrations to README.
Only use one worker when running up in development.
renamed 'index' import in bin/db-migrate to 'migrate'.
Factored mysql.migrations out into a separate lib/migrations.js file.
Merge pull request #532 from toolness/migrationify

Initial integration with node-db-migrate.