Added top-level convenience Makefile

* Supported targets: linux (default), macos, win32, clean

Makefile

+default: linux
+
+linux:
+	$(MAKE) -C src
+	$(MAKE) -C example
+
+macos:
+	$(MAKE) -C src
+	$(MAKE) -C example
+
+win32:
+	$(MAKE) -C src -f Makefile.mingw
+	$(MAKE) -C example -f Makefile.mingw
+
+clean:
+	$(MAKE) -C src clean
+	$(MAKE) -C example clean