NORM protocol extension

This includes the addition of the "norm_engine.hpp" and "norm_engine.cpp" files with minor changes to the "session_base.cpp" and "socket_base.cpp" files to recognize the "norm" transport type and instantiate the "norm_engine" as appropriate.  The "configure.ac" and "src/Makefile.am" files were also modified to add a "--with-norm" build option for libzmq.

Note the "norm_engine.cpp" code has calls to the "encoder::has_data()" method that has been removed since earlier libzmq versions (e.g. 4.0.3) and I have a reinstatement of this method in a version of "encoder.hpp" in my repository.  This outstanding issue should be discussed and resolved for incorporation of the NORM protocol extension into libzmq.