Remove redundant test.
Refactor: remove some duplicate code.
Extract method _set_args_for_current_signature (also, convert tabs to spaces).
Extract method _set_return_for_current_signature
Extract method _set_throws_for_current_signature
Add 'explain why' comments.
Convert tabs to spaces, remove trailing spaces.
Bug fix: duplicate expectations with no args, returns, or throws not being detected.

Moved the duplicate check from the args(), returns(), and throws()
methods to the expectation methods.
Extract method _start_new_signature
Extract method _add_method_to_mock
Add Vagrantfile.