Addition of an OSX .app file of your wonderful mongo client

[dmalin]

I had been using your mongo client for a bit and got tired of having to navigate to the folder to open the jar. I couldn't figure out how to make a shortcut easily so i figured why go one step further. So I generated a full fledged osx .app file. I figured other OSX users who used your great client (by far the most useful I have found) woud appreciate this.

[agirbal]

thanks for contributing. I will change some of the fields and make it part of the packaging process for next version