OSX launchdaemon and homebrew formula

***Added functionality***
- launchdaemon
- homebrew formula

***Description***
The OSX launchdaemon has to be moved to `/Library/LaunchDaemons/`. As a result the syncany daemon will be launched once the system is started.

The homebrew formula allows less experienced users to use syncany without having to manually build it. Homebrew only accepts production ready software, therefore we need to provide the formula. (thanks to
@tvon, closes syncany/syncany#267).

***TODOs***:
- [x] Add gradle operation to move the `org.syncany.daemon.plist` file to ~~`/Library/LaunchDaemons/`~~ `/Library/LaunchAgents/` (see: [Creating Launch Daemons and Agents](https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html))
- [ ] Create a redirect such as `http://syncany.org/r/homebrew` (make sure the downloaded file is called `syncany.rb`; homebrew convention)