Add all missing API commands

This pull request does a few things. Please let me know if you'd prefer some changes to not be made and I can open another pull request.

* Update the dev environment to use bundler (if you like) or gemspec. The rspec and doc used were so out of date that I had to do this to get the original test suite working for me.
* Use Litmus::Base.configure instead of Litmus::Base.new for configuring the base class.  This is just a style preference for me as it didn't feel right to create an instance to configure the class.  Both ways works for backwards compatibility.
* Add .retest and .update to the Result endpoint 
* Add .list, .create and .show to a new Report endpoint

Specs and readme updated as well.