Achieve 100% test coverage

[kadamwhite]

The provisional roadmap for 1.0 (#73) includes 100% code coverage. This issue exists to make sure that, prior to tagging that release, we do in fact have said coverage.

[tbranyen]

Lets take this one step further and get 4.0 on Code Climate as well. Currently we're held back by some complex methods in lib/tree.js