Conversion to ActiveRecord

There are still 4 specs failing so this is by no means a complete pull request. There is also a lot of mongo specific code simply commented out at this stage.

I'm pretty sure the reason the last 4 specs are failing is because of this line https://github.com/robjacoby/swordfish/pull/new/activerecord-migration#L18R8 but I'm not sure what to change it to as I'm not very familiar with rspec mocks.

If someone could point me in the right direction I'm happy to clean up the rest, ready for merge.