Server config schema

[rpavlik]

The server config schema is right now implicitly defined in

• https://github.com/OSVR/OSVR-Core/blob/master/inc/osvr/Server/ConfigureServerFromFile.h
• https://github.com/OSVR/OSVR-Core/blob/master/inc/osvr/Server/ConfigureServer.h
• https://github.com/OSVR/OSVR-Core/blob/master/src/osvr/Server/ConfigureServer.cpp

If it's explicit, that's both documentation, and useful for generating a browser-based editor.

[rpavlik]

Initial work on this is ready to use.