Refactor volumeUp/volumeDown

This also has the advantage that Spotify no longer needs to be activated and
the frontmost window. All can happen in the background now.
Refactor apple script a bit

Refactored the apple scripts of some functions to fit into one line without
sacrificing readability. This wastes less vertical space and should be easier
to understand.
Merge pull request #2 from rmehner/better_volume_control

Refactor volumeUp/volumeDown
Merge pull request #3 from rmehner/refactor-apple-script-a-bit

Refactor apple script a bit