Merge yagni-test and yagni repositories

[venantius]

This has been bothering me for a while, but I'm starting to get to the point where it's really annoying me. Maintaining separate directories for the test project and the main repo screws with the whole point of having CI set up; it would be a much better idea for me to maintain both directory structures in a single project.

Of course, now that I say that I can't find a valid example of the directory structure I want. Hm.

I think a starting point would be to split this project into two root directories with two separate projects: yagni, and lein-yagni. That way I can test the former cleanly without hitting, say, Leiningen errors. Then lein-yagni can run tests against the associated version of yagni. I don't see a lot of good examples of actually testing plugin behavior v. library behavior but at a bare minimum checking to see that leiningen is still capable of running with the plugin on the path is a good start.

[venantius]

Ref. #20