Added mutex and semaphore modules to demonstrate locks

Added two modules to demonstrate locks.

Mutex demonstrates a simple mutual exclusion lock. Semaphore
demonstrates a semaphore for controlling access to a pool of resources.

The main class of both programs is App.java.

pom.xml

@@ -123,6 +123,8 @@
 		<module>feature-toggle</module>
 		<module>value-object</module>
 		<module>monad</module>
+		<module>mutex</module>
+		<module>semaphore</module>
 	</modules>
 
 	<dependencyManagement>