Add unit tests

As discussed on IRC, here are some tests written with atoum. 
I will continue to cover Pickle source code.

Build status: [![Build Status](https://travis-ci.org/jubianchi/pickle.svg?branch=atoum-tests)](https://travis-ci.org/jubianchi/pickle)
@stof the vendor bins are excluded by the `.gitignore` file so I don't think it's a problem to put them all in the same folder.

It's seems easier to use when everything is at the same place but if you know potential issues with this, I'll fix it :)