Added rapidjson library

Added web_interface_utils.cpp/.h to common to provide string based functions for outputting JSON
Adjusted CMake to include rapidjson library and new web_interface_utils.h/cpp to common

Currently, I have code set up hackishly for testing so do not think it is final. It is merely for demonstrating what I've done and played with.

common/CMakeLists.txt

@@ -89,6 +89,7 @@ SET(common_sources
 	tinyxml/tinyxml.cpp
 	tinyxml/tinyxmlerror.cpp
 	tinyxml/tinyxmlparser.cpp
+	web_interface_utils.cpp
 )
 
 SET(common_headers
@@ -232,6 +233,7 @@ SET(common_headers
 	StackWalker/StackWalker.h
 	tinyxml/tinystr.h
 	tinyxml/tinyxml.h
+	web_interface_utils.h
 )
 
 SOURCE_GROUP(Patches FILES