Add skipCleanUrls metadata option
Shorten the skipCleanUrls option to cleanurls: false, clean up the README, add tests
v2.5.2. Improvement.

- v2.5.2 October 10, 2013
	- Optionally skip using cleanurls on a document based on a metadata switch
		- Thanks to [Erv Walter](https://github.com/ervwalter) for [pull request #6](https://github.com/docpad/docpad-plugin-cleanurls/pull/6)