doco!
Bump simplecov to 0.7.1.

0.6.4 for some reason was causing my bundler to not find the right
version of the simplecov-html dependency, despite it being installed.
Make ./exe/install_phantomjs work for OSX.

It just installs it via brew, but pins the brew recipe at 1.7.0.

It is entirely plausible that the latest version would do, but I don't
have proof.
Removed line about pending tests
Merge pull request #150 from seomoz/phs/use-ruby-version-file

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