Adding Notifications for OSX

Sometimes you are programming and you are not tuned on your email inbox, so I implemented a feature on the shell and python versions now check if the running Operational System is a Mac OSX (Darwin kernel), and if do so, it pops up an OS notification using the notification center, just like this:

![image](https://f.cloud.github.com/assets/928035/1461746/3ca7b5aa-44c9-11e3-9aab-9d0d6d626155.png)