Define DocType on render.

Added all doctypes as per http://www.w3.org/QA/2002/04/valid-dtd-list.html to Document class.
Added references to HTML type doctypes from Document class constants to Html class for convenience.
If doctype is supplied replace current doctype if it exists before prepending doctype.
Added some comments for completion.
Another approach at replacing the doctype.

It appears that we can loadHTML the <!DOCTYPE and then use name, publicId and systemId to populate the DOMImplementation->createDocumentType().
This doctype can be used to replaceChild of the current document's doctype.
Returning saveXML formats the html5 empty doctype better and it is much less nurve wrecking to preg_replace the <?xml version="1.0" encoding="iso-8859-1" standalone="yes"?> than a doctype.
Both this and the previous commit solves the problem, I might lean towards this option more.
Fix thumb print.

The buck might as well stop here. Attempt to keep transparency with contributors list, thanx goes to Henrique Moody. It's a honour to be on board.
Merge pull request #24 from nickl-/render-doctype

Define DocType on render.