#22 Generic C++ server class

[Roman Stepanov]

This patch adds a generic C++ XML-RPC server class,
which allows to build transport independent XML-RPC
server. Incoming requests must be passed to server as
std::string strings, server replies passed back also as
std::string. This can be used in debugging and also
very useful if you want to manage connections and HTTP
headers by yourself.

Also this patch adds more const-awareness to C
interface. You must apply patch 501463 (Improve C++ API
const awareness) before trying to apply this one.

[Roman Stepanov]

Generic C++ server class and C interface const awareness