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)
I think I should revert this line:

* With my patch, it says "Give me the current version of the package, the one that is currently being developed"
* Before my patch it said "Give me the last released version of the package"

Which one is good ?