Implement a registry of renderers.

This is now the soruce of renderers as used in DefaultPageImpl::get().
Renderers are identified by ctype (with a ctype of "default" added for
the plain HTML renderer), and there are two types: stream and page.

 - Stream renderers produce an InputStream which is sent to the client
   through a StreamView with the appropriate content-type.

 - Page renderers produce a String which is inlined into a JspView,
   with the content-type discarded.

 - If there are multiple renderers for the same ctype (why you would do
   that, I don't know), stream renderers are preferred to page
   renderers.