Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Session base class now handles the engine events exclusively. It notifies
derived session types using dedicated "attached" and "detached" events.

Couple of bugs was fixed along the way.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

C and C++ headers moved from bindings/ to include/, bindings/ removed
--with-c and --with-cpp options to configure removed, C and C++ now built
and installed by default

zmq_encoder/decoder are able to add/trim prefixes from messages; fair queueing and load balancing algorithms factorised into separate classes