Issue #72: Added mongodb store.

This is to address [Issue #72](https://github.com/twitter/storehaus/issues/72)

Added basic support to use MongoDB as storage engine. Only supports `get` and `put` now.

Use MongoDB's [casbah](https://github.com/mongodb/casbah) driver, will upload a different PR for [ReactiveMongo](https://github.com/lanxx019/ReactiveMongo)

To run the test, you need a `mongod` instance running on `127.0.0.1:27017`. You can run the following command to start a `mongod` instance for this:

```shell
$ mongod --port=27017 --dbpath=/data/db --logpath=/data/mongod.log --logappend --smallfiles --nojournal --oplogSize=100 --nohttpinterface
```

Please pin me if you have any trouble running the tests.