Fix Sound Effects on Linux

[chanceVermilion]

First off I want to tell you that I just found this project and I'm impressed. I haven't played Osu! in years and in the mean time I've made Ubuntu my OS of choice so I was happy to find a native way to play the game again.

This fixes (on my Ubuntu 16.04 machine) #89 . Of course that fix introduced another issue on the Download Menu which I fixed by removing what looks like a redundant call to stop the track. I also did a directory restructure to make the project more maven-y, added a test, and changed the exec plugin to run during the integration-test phase. I've played around with this on one machine and now the sound effects and song download previews work.

[itdelatrisu]

Thanks a lot for figuring this out! (Especially since I never managed to reproduce any of the audio issues on Linux...)

I committed just the audio fix in 039dd66 (didn't want to bother you by asking to not move around imports, etc.).

I'm slightly against the other changes for these reasons:

• Not really a fan of Maven's project structure with all the useless (for this project, at least) nested directories. I think it's plenty clear enough as is, too.
• At this point it seems kind of hopeless/not worth the effort to add unit tests, and I don't think it makes sense to have a testing framework when there will inevitably be almost nothing added to it. (Your test also wouldn't have failed on my (Windows) machine anyway.)
• I realize this is a pretty sketchy use of mvn compile, but exec isn't really an "integration test" either. Not sure if there's a better place to put it. :/

Closing this PR since the main issue was addressed -- feel free to keep commenting if you disagree with anything I said.