add basic test suite using vimrunner

Hi, I finally got around to implement some basic tests. I ended up using [vimrunner](https://github.com/AndrewRadev/vimrunner). This has one failing test that works using my other pull request (#8). I will update the other pull request when this one is merged. I may also add further tests but wanted to get the basics merged first.
To run the tests use `bundle install` and then `bundle exec rspec`.

The failing test case is "indent by one level when after a '(' that is at the end of its line".

I renamed the function to `GetPythonPEPIndent` so I can check that our version is used instead of the vanilla implementation that is also called `GetPythonIndent`.