[WIP] atoum-phpunit bridge

This PR is a work in progress. It aims to implement a bridge allowing atoum to run PHPUnit tests.
Currently, this supports the following from PHPUnit:

## Assertions

<table>
<tr>
<td>assertFalse</td><td>assertTrue</td><td>assertNull</td><td>assertNotNull</td>
</tr>
<tr>
<td>assertEquals</td><td>assertNotEquals</td><td>assertContains</td><td>assertNotContains</td>
</tr>
<tr>
<td>assertSame</td><td>assertInstanceOf</td><td>assertNotInstanceof</td><td>assertArrayHasKey</td>
</tr>
<tr>
<td>assertCount</td><td>assertFileExists</td><td>assertFileNotExists</td><td>assertStringStartsWith</td>
</tr>
<tr>
<td>assertAttributeEquals</td>
</tr>
</table>

## Mock

<table>
<tr>
<td>getMock</td><td>getMockForAbstractClass</td><td>getMockBuilder</td>
</tr>
</table>

### Expectations

<table>
<tr>
<td>expects</td><td>any</td><td>exactly</td><td>once</td><td>atLeastOnce</td><td>at</td><td>never</td>
</tr>
<tr>
<td>with</td><td>equalTo</td><td>identicalTo</td>
</tr>
</table>

### Definition

<table>
<tr>
<td>will</td><td>returnValue</td><td>returnVallback</td><td>onConsecutiveCalls</td><td>throwException</td>
</tr>
</table>

So basically, something like that will work with atoum:

```php
<?php
$mock = $this->getMock('\\Foo\\Bar');
$mock
    ->expect($this->any())
    ->method('baz')
    ->with('arg1', 'arg2')
    ->will($this->returnValue('returnval'))
;

//...
```

I ported two test suites while writing this:
* Composer
  * jubianchi/composer: https://github.com/jubianchi/composer/compare/composer:master...master
  * travis builds: https://travis-ci.org/jubianchi/composer
* Twig
  * jubianchi/Twig: https://github.com/jubianchi/Twig/compare/fabpot:master...master
  * travis builds: https://travis-ci.org/jubianchi/Twig

I'm still working on this as there are some bugs to fix (like the one making composer's build fail on ```5.4``` and ```5.5```)

Feel free to review everything, perhaps test it, and let me know what you think about this tool ;)