Implemented Simple Backend

I implemented a simple backend. There are no unit tests for it (as none of the backends are unit-tested). I'm going to raise another issue about that elsewhere.

Please test the following things:

- [x] Gzip support works
- [x] Alternate root paths work
- [x] `writeToDisk: false` turns it into a memory-only store.
- [x] You can save models
- [x] You can remove models
- [x] You can query models (`find`, `findOne`)
- [x] You can update models ('findOneAndUpdate`, `update`)
- [x] You can perform map/reduce queries.

Raise any issues here.