"make mac-app" command

**Note:** See comments in issue cryptocat/cryptocat#382 ("Mac App Distribution outside of the Mac App Store").

* This `Makefile` command generates a Cryptocat.app without having to open Xcode. Also zips it (same as with the browser plugin make commands). (Comment in cryptocat/cryptocat#382 notes that this could be helpful for automating builds & easily providing distribution outside App Store.) 

1. Copies the resources (exactly like `make mac` does)
2. Runs the `xcodebuild` shell command (which is like opening Xcode and pressing "build") to clean & rebuild the app, but with flags to place the .app in the /release/ directory.
3. Puts the .app inside a .zip file, like the browser plugin `Makefile` commands do.

*Might* not work if you have already been using `Cryptocat.xcodeproj` and you’ve customized your build schemes or other bits about your Xcode environment for this project.

But I’ve tested this on a fresh checkout and it works great. See comments below for example checkout & build output.