Introduce support for asynchronous server-side methods (#12)

* Add preliminary changes for async server methods

* Refactor the Message concept and break it into distinctive types

* Continue working on async server methods (high-level API mainly)

* Continue developing support for async server

* Finishing async server methods

* Finishing async server methods (fixing tests & cleaning up)

* A little code cleaning

* Add unit tests for type traits of free functions

* Support for generating async server methods in stub headers

* Update ChangeLog for v0.3.0

* Update the tutorial with how to use async server-side methods

* Update the TOC in sdbus-c++ tutorial

* Update numbering in TOC

* Remove unnecessary code

* Final cleanups

include/sdbus-c++/IObject.h

@@ -34,7 +34,7 @@
 
 // Forward declarations
 namespace sdbus {
-    class Message;
+    class Signal;
     class IConnection;
 }
 
@@ -70,6 +70,28 @@ namespace sdbus {
                                    , const std::string& outputSignature
                                    , method_callback methodCallback ) = 0;
 
+        /*!
+        * @brief Registers method that the object will provide on D-Bus
+        *
+        * @param[in] interfaceName Name of an interface that the method will belong to
+        * @param[in] methodName Name of the method
+        * @param[in] inputSignature D-Bus signature of method input parameters
+        * @param[in] outputSignature D-Bus signature of method output parameters
+        * @param[in] asyncMethodCallback Callback that implements the body of the method
+        *
+        * This overload register a method callback that will have a freedom to execute
+        * its body in asynchronous contexts, and send the results from those contexts.
+        * This can help in e.g. long operations, which then don't block the D-Bus processing
+        * loop thread.
+        *
+        * @throws sdbus::Error in case of failure
+        */
+        virtual void registerMethod( const std::string& interfaceName
+                                   , const std::string& methodName
+                                   , const std::string& inputSignature
+                                   , const std::string& outputSignature
+                                   , async_method_callback asyncMethodCallback ) = 0;
+
         /*!
         * @brief Registers signal that the object will emit on D-Bus
         *
@@ -138,7 +160,7 @@ namespace sdbus {
         *
         * @throws sdbus::Error in case of failure
         */
-        virtual Message createSignal(const std::string& interfaceName, const std::string& signalName) = 0;
+        virtual Signal createSignal(const std::string& interfaceName, const std::string& signalName) = 0;
 
         /*!
         * @brief Emits signal on D-Bus
@@ -149,7 +171,7 @@ namespace sdbus {
         *
         * @throws sdbus::Error in case of failure
         */
-        virtual void emitSignal(const sdbus::Message& message) = 0;
+        virtual void emitSignal(const sdbus::Signal& message) = 0;
 
         /*!
         * @brief Registers method that the object will provide on D-Bus