make doc
[doc] Xs_client, Xs_client_unix: cosmetics
Updated doc title for Xs_client, Xs_client_unix
Improved doc for module Xs_protocol
Rewritten module Xs_handle to be more functional style, with better interface and documentation
Renamed with_xs/with_xst -> immediate/transaction (like in Mirage)
Make the xs_client_unix look more like the lwt one, renamed xs_client to xs_client_lwt for more consistency.
Updated CHANGES.
Merge pull request #6 from vbmithr/master

Mostly doc, rewritten module Xs_handle