Work around bug in autobuild 0.8 source_environment
Add installables for libjson, tinythreadpp, zeromq.

These do not yet replace the copies in the third_party directory, but they do
fetch and install properly.
Delete third_party subdir; build CEF wrapper with external packages.

Eliminate CMake logic to build third_party sources. Replace with directives to
find and use installed autobuild packages.

This requires bringing in an updated build of the 3p-zeromq library: at first
we didn't realize Mark had also dropped the C++ binding header zmq.hpp from a
different repo into the third_party/zeromq-4.0.4/include directory. That file
has now been added to 3p-zeromq.

Tweak CMake directives (and ZmqMonitor.cpp) to get a clean build under VS
2010. Perhaps Mark was getting builds because he was using VS 2013.
Use size_t instead of int for size params in SchemeExtenderI::read().

In addition to being more intuitive, this clears up signed/unsigned warnings
in the build.