expanded endpoint methods and wrapper objects

  * better endpoint class test coverage and fixture/mocking approach
  * friendly user-agent
  * more sane and scalable fixture mechanism
  * more better test coverage for endpoint classes
You're referring to the ``__version__`` thing? Pattern stolen from ``requests``: https://github.com/kennethreitz/requests/blob/master/requests/utils.py#L530

Looking at this, tho, made me realize that I'm building those headers/user-agent on every request, which is dumb. Updated to build once at client creation.