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)
defining a constant that is not uppercase and namespaced is a bit WTF to me :) FIXTURES_DIR is shorter and you immediately get what this is when you see it being used. 

If I read `file_get_contents(tests\fixtures . DIRECTORY_SEPARATOR . 'maintainers.xml')` below before having read this line I would really have wondered what the hell was going on.