Add custom test helpers to Assert and Should

Includes the growing list of test helpers into Assert and Should.

I exclusively use Assert and Should over $this->assertXXX, because it helps with readability. I also want to use the custom assertMethods included in the same package.

e.g.

```php
// $this->assertValid($model);
Assert::valid($model);
Should::beValid($model);
```

I tried to add tests for the changes, but I couldn't wrap my head around these (see gist: https://gist.github.com/felixkiss/5713116)
Please give feedback for the tests, so I can include them.