Add tools to dump logged certificates and STHs.

They are rather basic for now, but are useful when debugging servers using
the FileDB storage.

See also: https://codereview.appspot.com/142170043/
It's only used internally in the client (why, oh why?!?), but I'm dumping the content now, just in case. I'm base64-encoding it, as it seemed like the thing to do.