Port spotify mac script from tmux-powerline (revised)

I've modified @Xlator's PR https://github.com/Lokaltog/powerline/pull/699 with the modifications you recommended (removing color codes, moving the applescript methods out into it's own lib file, removing extra new lines after method).

I also got the segment working out of the box. I was having problems with it displaying (needed the color scheme definition).

Please review my code as I'm not a python developer. I have vim set up to remove trailing spaces and use two space indention. I noticed the contributing guidelines said tabs, so I took that into consideration when editing (double check me though).

I'm not familiar with flake8 or running test suites in python. My code is almost a direct copy of @Xlator's code with some minor changes/deletions so I think it's fine. I'd recommend setting up travis to run your test suite and other contributing tools/checks.

By the way, I tried getting the included spotify segment working but dbus is a pain on OS X. I got dbus, dbus-glib and dbus-python installed so that the script wouldn't complain the lib was missing but for some reason it would error at [`powerline/segements/common.py#L992`](https://github.com/Lokaltog/powerline/blob/a4e8f36f3/powerline/segments/common.py#L992). I didn't debug it any further though.