Add user notifications reporter which sends notifications to NSUserNotificationCenter after an action succeeded or failed.

When building or running tests in Xcode developer is able to see results of these actions even if Xcode is collapsed. When running xctool in terminal developer has to follow the progress and manually check from time to time if building/testing/etc is completed.

This changes bring to xctool support of Mac OS X User Notifications, which are available in Mac OS X 10.8 and higher. Whenever the action is completed appropriate user notification is sent.

Examples of notifications:
![xctool_notification](https://cloud.githubusercontent.com/assets/1044236/2771973/a26f356c-ca74-11e3-92d0-71809dcaa3df.png)
![xctool_notifiction_center](https://cloud.githubusercontent.com/assets/1044236/2771974/a2715306-ca74-11e3-9889-fa50607cc412.png)