Implement automatic JSON API generation

qmake file add jsonapi-generator target to compile JSON API generator
qmake files add rs_jsonapi CONFIG option to enable/disable JSON API at compile
  time
RsTypeSerializer pass down same serialization flags when creating new context
  for nested objects serial job
RsGxsChannels expose a few methods through JSON API as example
Derive a few GXS types (RsGxsChannelGroup, RsGxsChannelPost, RsGxsFile,
  RsMsgMetaData) from RsSerializables so they can be used for the JSON API
Create RsGenericSerializer::SERIALIZATION_FLAG_YIELDING so JSON objects that
  miss some fields can be still deserialized, this improve API usability
SerializeContext offer friendly constructor with default paramethers
Add restbed 4.6 library as git submodule as most systems doesn't have it yet
Add a bit of documentation about JSON API into jsonapi-generator/README.adoc
Add JsonApiServer class to expose the JSON API via HTTP protocol

retroshare-gui/src/gui/settings/WebuiPage.cpp

@@ -22,6 +22,9 @@ resource_api::ApiServerMHD* WebuiPage::apiServerMHD = 0;
 resource_api::ApiServerLocal* WebuiPage::apiServerLocal = 0;
 #endif
 resource_api::RsControlModule* WebuiPage::controlModule = 0;
+#ifdef RS_JSONAPI
+JsonApiServer* WebuiPage::jsonApiServer = nullptr;
+#endif
 
 WebuiPage::WebuiPage(QWidget */*parent*/, Qt::WindowFlags /*flags*/)
 {
@@ -105,6 +108,10 @@ QString WebuiPage::helpText() const
 // TODO: LIBRESAPI_LOCAL_SERVER Move in appropriate place
 #ifdef LIBRESAPI_LOCAL_SERVER
 	apiServerLocal = new resource_api::ApiServerLocal(apiServer, resource_api::ApiServerLocal::serverPath());
+#endif
+#ifdef RS_JSONAPI
+	jsonApiServer = new JsonApiServer(9092);
+	jsonApiServer->start("WebuiPage::jsonApiServer");
 #endif
     return ok;
 }
@@ -126,6 +133,10 @@ QString WebuiPage::helpText() const
         delete controlModule;
         controlModule = 0;
     }
+#ifdef RS_JSONAPI
+	delete jsonApiServer;
+	jsonApiServer = nullptr;
+#endif
 }
 
 /*static*/ void WebuiPage::showWebui()

retroshare-gui/src/gui/settings/WebuiPage.h

@@ -3,6 +3,10 @@
 #include <retroshare-gui/configpage.h>
 #include "ui_WebuiPage.h"
 
+#ifdef RS_JSONAPI
+#	include "jsonapi/jsonapi.h"
+#endif
+
 namespace resource_api{
     class ApiServer;
     class ApiServerMHD;
@@ -55,4 +59,7 @@ private:
   static resource_api::ApiServerLocal* apiServerLocal;
  #endif
   static resource_api::RsControlModule* controlModule;
+#ifdef RS_JSONAPI
+  static JsonApiServer* jsonApiServer;
+#endif
 };