[notmuch] regress option to messages iterator

[meskio at sindominio.net]

Two patches for implement the regress functions on messages. With them 
notmuch_messages_t can be use as an iterator forwards and backwards.

Up to now not really useful, nothing use them. I send them mainly to review.
I'll do the same to threads and tags, so the UIs can use them.

PS: It's the first time I'm using git-send-email, I hope I do everything well.