[feature] Define matchers on the fly

It should be possible to define matchers on the fly

``` php
\PHPSpec\Matchers\define('contain', function($match, $actual) {
    return in_array($match, $actual);
});

class MyArraySpec extends \Context\PHPSpec {
    function itAddsNewElementsUsingArrayOperator() {
        $array = new MyArray();
        $array[] = 42;
        $array->should->contain(42);
    }
}
```
