Added a slimmed down interface for the client

At first glance, the HipChatClient has a lot of methods and they aren't necessarily clear what they do. I added an interface IHipChatAPI that has a subset of methods to get started with.

Perhaps this could act as the start of a refactoring, if I get motivated. This may or may not be useful for your purposes or proper for inclusion, but I figured I'd send it along anyway.

Also, I removed some of the unused build configurations (x86 or x64 specific in lieu of Any CPU since that should be sufficient and simpler).