Allow the extension of the PhantomJS driver with custom JS includes:
*  example of how we would want to extend the environment
*  issue command to load extensions
*  browser command the page to load extensions with agent
*  load extensions into the env
*  configuration of extensions out of browser initialize
*  Update the README to match feature
*  allows single files to be used too
Merge pull request #188 from JonRowe/extend_the_env_with_js

Allow extension JS files to be injected into PhantomJS with the agent