Fixed find operations where "id" is the key in MongoDB.

Hey Evan,

Unlike other dbs, mongo is using "_id" for its record ids, so if I wanted to find a group of records from a list of ids, I'd need to write code specifically for mongoDB to do something like this:
```erlang
boss_db:find(model, {'_id', 'in', ModelIds}).
```

I committed a "fix" which, when you are doing a `find` with conditions, just translates any key called 'id' to '_id'.  This way, my `find` operations are consistent across dbs.

Thanks!
-Lucas-