Add support for enough opcodes to read Graphite's Rendering stream

I'm trying to write a proxy that intercepts graphite rendering commands.  I
need to be able to read and write pickle streams that look like
http://graphite.readthedocs.org/en/0.9.10/render_api.html#pickle . This patch
(sorry it's just one giant one) adds support for enough opcodes to unpack a
graphite rendering pickle.

I also need to add some simple encoding support, but I'm not sure how involved
I'm going to get with that.