build also a jar file

Changed the configuration of the maven-war-plugin: added
properties so that an additional jar file named
connectivity.proxy-classes.jar is built. This makes it
possible to include the proxy servlet into other maven
projects.

Maven pom.xml dependency snippet:
<dependency>
	<groupId>com.sap.cloudlabs.connectivity</groupId>
	<artifactId>connectivity.proxy</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<classifier>classes</classifier>
</dependency>