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)
No big deal, but using DIRECTORY_SEPARATOR is really useless. '/' works everywhere. I think it's better not to use it since it keeps the source easier to read.