Mac nightlies: back and improved!

Thanks to the awesome work of mokaspar, the mac nightlies are now back!

The new build system checks every 15 minutes if there's a new commit and if available, it compiles and publishes the result on http://prereleases.musescore.org/macosx/nightly/. Building takes some time but if everything goes well max 2 hours after commiting, a build is available to test the changes.

Of course, nightly builds are meant for testing and are not suitable for serious work. A file created with one nightly could be unusable with a later one. That being said, you can test as much as you want and discuss issues in this forum. When an issue is isolated, you can post it to the issue tracker. As a reminder, read Freelinking: Unknown plugin indicator .