Added new waypoints module

I needed similar functionality to what the jQuery waypoints module provides in angular. I suppose I could have used ui-jq and used waypoints itself, but I liked the idea of having a native angular way to do this, so I wrote a module to do it. I used many of the same conventions as are used in the ui-scrollfix plugin, it's just that instead of assigning a class, you get callbacks.

If it's interesting to this project, feel free to take it! If the idea is interesting in general but you need some additions/changes, I'm happy to oblige. 

Thanks,

-Matt