Add prefixes to category method names

It's often [recommended][1] (although far from widely agreed upon) to prefix category method names to protect against name collisions. I'm not sure if the absence of prefixes in the existing code was intentional or not, so if it was, please feel free to ignore this PR. I made the change for myself anyway, so I figured I may as well offer it to you.

[1]: http://nshipster.com/namespacing/