Add possibility to test blade views

[helloiamlukas]

This pull request allows the use of the assertHtml5, assertElementExists and assertFormExists functions on Illuminate\Testing\TestView objects.

This adds the possibility to test Blade views. Example:

$this->view('navigation')
    ->assertElementExists('nav > ul', function(AssertElement $ul) {
        $ul->contains('li', [
            'class' => 'active',
        ]);
    });

[sinnbeck]

Sorry for the late merge. But thanks for the very thorough PR with nice features and tests!