feat(manage_webdriver): redo the script to run and install selenium/webdriver

Breaking Change.
As outlined in Issue #296, redoing the way the selenium/webdriver
install and run helper scripts work. Now, the 'manage_webdriver' script
will be available either locally or globally (depending on how protractor
was installed). It replaced install_selenium_standalone and the 'start' script
that was provided after install. Run `manage_webdriver update` to download
new versions of selected webdriver binaries. Run `manage_webdriver start`
to start the standalone server.