added a html render factory for html strings
* refactored `string_render_factory` into `abstract_renderer_factory`, which accepts and uses a `default_content_type`
* this makes the following 2 render possible, but also makes it trivial to create new renderers that accept strings and do the correct content-type setting
** `string_renderer_factory` returns `abstract_renderer_factory(info,default_content_type='text/plain')`
** `html_renderer_factory` returns `abstract_renderer_factory(info,default_content_type='text/html')`