add simplecov, update README, continue to test

main focus right now is improving test and documentation coverage.
simplecov will generate coverage reports on every `rake spec`. I'm
attempting more or less complete coverage before I move forward.
Specifically, I know I need to deal with the objects returned by
`HieraController#lookup` and friends (`#lookup_additive`, `#get_all`,
etc) because they are actually quite ugly. The main issue there is that
my tests don't really cover the UI, although it may be as simple as
updating my cucumber tests? I'm not 100% sure yet.