Also add missing priority commands (prio and prioid)

Mention explicitly py2 is not supported

python-mpd now deals only with unicode

* added searchadd, searchaddpl, seekcur commands
* new client to client commands

Only supports python >= 3.2

Iterators are considered safe now, as all programming errors now raise an
exception instead of breaking horribly.  No need to scare people away from
it anymore.