Add some informative terminal output for guiding the user through the installation process
Add support for recursively traversing a directory of `_attributes.yml` files and merging them into one
Update the default rbenv Ruby version to `2.1.4`
Update the installer so that it responds properly to changes in the default rbenv Ruby version
Add the `update` subcommand to the `osx-bootstrap` executable for running the installer with its original command-line arguments