Build an additional jar file to allow other projects to use the proxy servlet by inclusion.

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>