libresapi: added channels/create_post

group creation acknowledge not implemented yet

usage:
$ curl --data "{\"group_id\":\"<group_id>\",\"subject\":\"just a test\",\"message\":\"test message\"}" http://<host:port>/api/v2/channels/create_post

parameter object:
{
    group_id: required string,
    subject: required string,
    message: required string,
    thumbnail_base64_png: optional string,
    files: optional array of
        {
            name: required string,
            hash: required string,
            size: required number
        }
}

libresapi/src/api/ChannelsHandler.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include "ResourceRouter.h"
+
+class RsGxsChannels;
+
+namespace resource_api
+{
+
+class ChannelsHandler : public ResourceRouter
+{
+public:
+    ChannelsHandler(RsGxsChannels* channels);
+
+private:
+    ResponseTask* handleCreatePost(Request& req, Response& resp);
+
+    RsGxsChannels* mChannels;
+};
+
+} // namespace resource_api