Added API to set the piece priority.
Added API to set the piece priority.

tchoulihan/frostwire-jlibtorrent,aldenml/frostwire-jlibtorrent,frostwire/frostwire-jlibtorrent,gubatron/frostwire-jlibtorrent,aldenml/frostwire-jlibtorrent,gubatron/frostwire-jlibtorrent,frostwire/frostwire-jlibtorrent,aldenml/frostwire-jlibtorrent,tchoulihan/frostwire-jlibtorrent,gubatron/frostwire-jlibtorrent,frostwire/frostwire-jlibtorrent,tchoulihan/frostwire-jlibtorrent