add parallel

From #668 

Setup:

Make sure your database.yml looks something like this:
```
test: &TEST
  adapter: mysql2
  encoding: utf8
  reconnect: false
  database: reservations_test<%= ENV['TEST_ENV_NUMBER'] %>
  pool: 5
  username: root
  password:
  socket: /var/run/mysqld/mysqld.sock
```

```
rake parallel:create
rake parallel:migrate
rake parallel:prepare
```

Then to run tests (don't use Spork, as parallel is incompatible: parallel is still faster though, plus once we upgrade to rails 4.1 we'll be using spring anyway)
```
rake parallel:spec
```