Changed custom volume slider to MPVolumeView

* Fixes issue #13

But there is a problem with running the tests for iPad (not introduced with this commit, but should be solved anyway): `error: testPlaylistVisibility (BeamMusicPlayerViewControllerTests) failed: -[UIPopoverController presentPopoverFromRect:inView:permittedArrowDirections:animated:]: Popovers cannot be presented from a view which does not have a window.`

As i mentioned before i'm new to writing tests and i've got no idea why `viewController.view.window` is `nil` in this test.