add Chromedriver support

I came across this project while looking to replace some custom code in a grunt task doing similar work with downloading selenium drivers. The one exception is support for the [Selenium Chromedriver](https://code.google.com/p/chromedriver/).

This adds an options object that can be passed to specify that the chrome driver should be downloaded and used while launching selenium. The original API still works and nothing else is downloaded unless you opt in.

I'm looking to use this as a dependency for a [grunt task for running selenium tests with mocha and webdriver locally](https://github.com/wookiehangover/grunt-mocha-selenium), and expand it to support running tests against chrome.