Added md5 as a jinja filter - returns hex digest of input

As briefly discussed with mdehaan on IRC yesterday, here is a tiny patch to expose the existing MD5 (string - md5s() function) from ansible utils as a Jinja2 filter.

I cannot find documentation for the jinja2 filter extensions that ansible provides, so have not updated that.
However this sort of thing would be useful to document somewhere, but need to find the right niche.