Embed io.Writer instead of bytes.Buffer

Rather large commit:
 - Rename package to htmlwriter, as io.Writer is
   now the decorated type
 - Rename HtmlBuffer to HtmlWriter
 - Rename NewHtmlBuffer to NewHtmlWriter (and
   add io.Writer argument)
 - Update generators
 - Update tests
 - Remove WriteToResponse method, as it is now
   possible to write directly to a
   http.ResponseWriter