MongoMapper support

Refs #81

I used the latest version of the gem available at Rubygems, 0.12.0. Maybe it works with older versions, I didn't test it.

Also, I'm in no ways an expert with Mongo, Mongoid or MongoMapper. Mongoid and MongoMapper are quite similar so I pretty much copy-pasted, changed the syntax and then checked the remaining differences that the test brought up.

I didn't update the documentation, figured that could wait until the code is reviewed. For executing the tests the only thing needed is setting the environment variable `PA_ORM` to `mongo_mapper`, and to use it in an app just do `orm :mongo_mapper` in the config block. I haven't tested it on a real app yet but I'll probably do it sometime this weekend.