Add test dataset

Okay, here it is! A few notes..

* I'm using port 27020 for the Mongo daemon to avoid conflicts with local instances that run on 27017-27019 by default when Mongo is installed.
* The ``--smallfiles`` command reduces the pre-allocated size of the database from 64mb to 32mb, at the expense of limiting the total size of the database to 512mb. I think we can live with that. :)
* The mongodb directory where the collections were stored has to exist before connecting a daemon to it, I was having trouble getting git to track the folder and Stack Overflow suggest I had a dummy empty file.
* It failed on my Travis, I think pymongo needs to be added as a dependency to .travis.yml -- I'll amend this in the next commit.
Let's go with `image`/`images` for long and `im`/`ims` for short. I'll try to keep to that convention myself, but call me out on it if I don't!