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 don't know: some people like to have notifications on IRC, some other don't... 

Sometimes, it acts like flood when many people submit patches: in this case, we should remove it if it's too disturbing.