Failed strict error checking.

Error:
Message: Accessing static property Xiphe\SlimAssets::$assets as non static
Line: 254

I chose to make $assets a non-static property so that I could have multiple instances of the asset manager. Now I can do things like:

    $vendor->js('vendor/bootstrap');
    $application->js('main');

and render them both out as separate files (vendor files don't change as often).

I also fixed a typo in ensureFileExists.