it should be possible use $world->assert() instead of $world->d->assert() in step definitions

In order to use SimpleTest API functions, SimpleTest assertions and custom assertions in step definitions, it is necessary to extend the context object - provided as $world in steps - with methods from BehatWebTestCase. This happens in support/hooks.php where BWTC gets instantiated as $context->d.

However, this leads to syntax like $world->d->assertTrue(). We want to use $world->assertTrue() instead. Due to the lack of multiple inheritance in PHP though, it doesn't seem possible to extend the $context object directly.