Add all missing API commands

[rgraff]

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 The rspec and doc used were so out of date that I had to do this to get the original test suite working for me.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.

[rgraff]

Thanks for the quick merge and gem release