Adds a bare-bones cmake-server implementation and makes it possible
to start that with "cmake -E server".
Communication happens via stdin/stdout for now.
Protocol is based on Json objects surrounded by magic strings
("[== CMake Server ==[" and "]== CMake Server ==]"), which simplifies
Json parsing significantly.
This patch also defines an interface used to implement different
versions of the protocol spoken by the server, but does not include
any protocol implementaiton.
Tobias Hunger