Implemented Simple Backend

[Morgul]

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.

[whitelynx]

Had to make some fixes, but everything in the simple DB backend now has unit tests, and everything works except the gzip support.

[Morgul]

Fixed the gzip support, and the unit tests.