Enable rbenv, document and simplify(ish) bootstrapping a dev env for macs.

`.rvmrc` has been ditched in favor of `.ruby-version`.

The version of simplecov has been bumped in the gemspec, since there was apparently some odd version conflict with `simplecov-html`.

`./exe/install_phantomjs` now handles macs by installing 1.7.0 via homebrew.

A contributing section has been added to the README.