Add option to upload static sites with clean urls

Adds the option to upload static sites with clean urls like `http://example.com/my-page` instead of `http://example.com/my-page.html`. When **cleanUrls** is set to true, grunt-cloudfiles uploads html files (except index.html files) without their extension and sets the content-type header to `text/html` so browsers handle them correctly.

The cleanUrls option is based on this [blog post about hosting Jekyll sites on Rackspace](http://nicholaskuechler.com/2012/07/03/jekyll-with-clean-urls-hosted-at-rackspace-cloud-files/), but it is useful for other static site generators like Hyde as well.