Send notification to NSUserNotificaitonCenter after the 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 any action is completed appropriate user notification is sent.

Inspired by https://github.com/alloy/terminal-notifier. Some of terminal-notifier source codes are taken and modified. Appropriate copyright information is added to `xctool/xctool/UserNotifier.m`.

Examples of notifications:
https://dl.dropboxusercontent.com/u/6872784/tmp/xctool/xctool_notification.png
https://dl.dropboxusercontent.com/u/6872784/tmp/xctool/xctool_notifiction_center.png