Update / improve godoc

[liamsi]

As part of the spec writing work, it would be nice to update and improve the golang documentation.

Current state is here: http://localhost:6060/pkg/github.com/tendermint/iavl/

[silasdavis]

I can't get at your localhost :)

I think we should make a few more specific bitesized documentation tickets (e.g. document file) and then close this one in favour of them.

These ticket might be good a bedfellow with #62

[faddat]

Using godoc everywhere just makes sense.

[tac0turtle]

closing as we did a reafctor recently and are planning on doing another one soon