Add testing

[cseickel]

We really need to add a full suite of tests to this project, which can be run both locally and as a merge check. A handful of tests were created in #61 using plenary, which looks pretty good.

What needs to be tested?

• Every documented config option should do what it says
• Every bug fix should include a regression test (maybe just for future fixes?)
• All variations of vim commands (:NeoTree...) should do what the documentation says.

[levouh]

I'll be working on #61 this weekend, and with the new source design it should be pretty trivial. Right now that one focuses on unit tests, however you can do functional tests with plenary's version of busted as well.