Close #19 Ronn advertises itself.

Ronn has advertised itself in Roff and HTML output (via the default Mustache template) for quite
awhile.  Roff output was tested for "generated with", this commit adds a tiny test on the HTML
output as well.
Merge pull request #78 from richardkmichael/test-for-advertising-in-html

Close #19 Ronn advertises itself.