Solution: update for 2015

Fixes #1320

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.

Metadata are built in stream_engine now.
This makes it easy to extend metadata with user-defined properties.

The get_credential () member function returns
credential for the last peer we received message for.
The idea is that this function is used to implement user-level API.

RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.

- designed for TCP clients and servers
- added HTTP client / server example in tests/test_stream.cpp
- same as ZMQ_ROUTER + ZMQ_ROUTER_RAW + ZMQ_ROUTER_MANDATORY
- includes b893ce set ZMQ_IDENTITY on outgoing connect
- deprecates ZMQ_ROUTER_RAW