Refactored the renderers to properly separate the text renderer from the base renderers.

The output parameter is now a global session parameter and the renderers use it to write their output on. Similarly the renderer parameter can now only be passed in the session (session.SetParameter("renderer", "TextRenderer").

BUG=
R=parki.san@gmail.com

Review URL: https://codereview.appspot.com/103090045