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)
The idea here is to show the information fetched from the package.xml.Not sure what you mean by "current version" vs "the last release".

This parser is used only once, to convert package.xml to the json&co format, all other following commands won't use it.