Sync with libstrophe 0.9.x

[pasis]

libstrophe-0.9 updates API (see ChangeLog file). Would be nice to reflect it in lib.rs.

Also is there a way not to implement structures like xmpp_ctx_t, xmpp_conn_t, etc? The idea is that user always work with pointers to those structures and doesn't have access to their internals. libstrophe may change fields in any release. Maybe rust supports forward declaration? Or you can leave all structures empty, I suppose it won't change the behavior.