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)
@pierrejoye as far as I understand, `$sx->version->release` points to the current release (not released yet) and `$sx->changelog->release->version->release` points to the last release in the changelog (the last released one).

I don't know if we can consider the release in `/version/release` as being a stable release and show its value as the package current version or if we should use the last one in the changelog.