"make mac-app" command. see issue cryptocat/cryptocat#382

Generates a Cryptocat.app without having to open Xcode. Runs xcodebuild shell command under the hood. May not work if you've already been working with Cryptocat.xcodeproj and you've customized your build schemes at all.
remove unnecessary var
Merge pull request #387 from mtigas/master

"make mac-app" command