Custom return value delegate sequences for variadic functions (#63)

* Custom return value delegate sequences for variadic functions
* Added unit tests for variadic functions custom return value delegate sequences
* Fixes in code style
* Variadic functions custom delegates also tested in C++
* Fixed some compilation warnings
* Added test for variadic function custom delegates sequence reset
* Updated documentation with variadic functions custom delegate sequences
* Update README.md
* Minor style changes.

Thank you very much @oliviera9!

test/fff_test_global_cpp.cpp

@@ -15,6 +15,8 @@ public:
         RESET_FAKE(voidfunc2);
         RESET_FAKE(longfunc0);
         RESET_FAKE(voidfunc1outparam);
+        RESET_FAKE(voidfunc3var);
+        RESET_FAKE(valuefunc3var);
         FFF_RESET_HISTORY();
     }
 };