Added HUDMessage type and addHUDMessage() method.
This merge request contains an interface to the HUDMessage class, which can be used to display messages such as achievements, item-get notifications, and general text notifications. (Example: http://i.cubeupload.com/AOJEWT.png). It also adds an "AddHUDMessage()" function to Root, which hooks into Game1's similarly-named function.
I tried to match the coding style of the surrounding code, but since this is my first merge request, please let me know if I did anything wrong and I will correct it! Thanks~