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:

- [ ] Gzip support works
- [ ] Alternate root paths work
- [ ] `writeToDisk: false` turns it into a memory-only store.
- [ ] You can save models
- [ ] You can remove models
- [ ] You can query models (`find`, `findOne`)
- [ ] You can update models ('findOneAndUpdate`, `update`)
- [ ] You can perform map/reduce queries.

Raise any issues here.