Prevents App Nap on macOS 10.9+

[albertosottile]

Solves #116 and #60 (finally).
Adds a new dependency: appnope (GitHub), required only on macOS. It is available on pip. If merged, I guess the installation instructions have to be updated accordingly.

(Note: please be patient if I did something horribly wrong since this is my first pull request.)

[Et0h]

Sounds great. Thanks for your contribution. If people can test this and let me know if it works and suggest appropriate changes to the instructions then that would help move things further forwards :)

[albertosottile]

Sorry, I had to close this briefly because I did something wrong in the fork. Now everything should be fine. I am open for questions if anyone wants to test this.

I can update the install instructions if you want, but I did not useconda. I am afraid we should switch to homebrew because it has the patched qt4 recipe needed for macOS 10.12. If you want, I can write some instructions that use homebrew.

Once this issue is fixed, we can start testing the .app packaging code. Thanks again.

[albertosottile]

I wrote a post with detailed instructions for the installation of Syncplay on macOS 10.8+ here:
http://www.albertosottile.it/syncplay/2017/09/08/install-syncplay-on-macos-10-8/

If you want, you can link it on the syncplay.pl website. I encourage everyone who wants to use Syncplay on macOS to follow these instructions so that we can test if this patch works.

[Et0h]

Cheers. The Syncplay website has been updated to link to your instructions. As the commit has now been merged the instructions should be updated to use the official Syncplay repo.

[albertosottile]

Done. I guess that issues #60, #112, and #116 should also be closed now.