Changed custom volume slider to MPVolumeView

[DerPipo]

• Fixes issue Add option to change audio route #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.

[DerPipo]

Which also means that the Travis CI build only tests for iPhone, i think. Could be another issue.

[HBehrens]

Thank you, Pipo. Before I merge this, could you do some adjustments?

• Creating an MPVolumeView in InterfaceBuilder is possible by placing a UIView and manually set its class in the inspector. This would simplify the code. I did sth similar with the transparent toolbar in the bottom, please have a look.
• Since MPVolumeView is not supported in the simulator, do you think a label "no sound control in simulator" or the like would make sense?

I will have a look at the unit-test issue, hence I created #16

[HBehrens]

Thanks :)