Unit tests with PHPUnit and Composer

Hello, 

Here is a proposal to manage unit tests. 

As unit tests are not part of the distribution, they don't suffer from the same constraints (PHP version, environment..). 

This is a developer stuff, so this is why I decided to manage them using modern PHP :
* Require PHP 5.3 +, uses namespaces
* [Composer](http://getcomposer.org/), to manage installation of libraries, including PHPUnit
* [Faker](https://github.com/fzaninotto/Faker), [Mockery](https://github.com/padraic/mockery) as utilities, and why not [Alice](https://github.com/nelmio/alice) ?

<pre>
$ cd tests
$ curl -s https://getcomposer.org/installer | php
$ php composer.phar install
$ vendor/bin/phpunit
</pre>

Right now I have written few test cases for utility methods, but this pull request aims to simplify writing & running unit tests for other contributors. 

This is a long road, but having unit tests is becoming mandatory now that there are new contributors and supposedly more code changed. 

I also have some utilities to stub the context & the configuration that I created in another repo. 

<pre>
PrestaShop\Test\Runtime :: configuration(array(
    'PS_CURRENCY_DEFAULT' => 1
));

PrestaShop\Test\Runtime :: context(array(
    'currency' => array(
        'id_currency' => 1,
        'conversion_rate' => 1.000000
    )
));
</pre>

I have never done that, but it should be possible to configure [Travis](https://travis-ci.org/) to automatically run the tests. 

Hoping you will like it !