Refactor isRunning

After more time with AppleScript I found a lot nicer method of determining
whether Spotify is running or not.

This shrinks `isRunning` down to one, easy-to-understand line, using `running`
attribute of the application Spotify. Apparently every application has a
`running` attribute, which I didn't know about before.
Merge pull request #12 from mrnugget/refactor_is_running

Refactor isRunning