[#18] exception filters can now share list of missing/validated artifacts

 * the list is expected to be standard Spring <util:list> bean. The type
   of elements needs to be String
 * Example of the list definition in XML:
       <util:list id="droolsjbpm-boms" value-type="java.lang.String">
           <value>org.drools:drools-bom:pom:.*</value>
           <value>org.jbpm:jbpm-bom:pom:.*</value>
       </util:list>
 * The list is then referenced like this:
       <filter:dependency-not-found missing-artifact="org.drools:.*:jar:.*">
           <filter:validated-artifacts ref="droolsjbpm-boms" />
       </filter:dependency-not-found>
Merge pull request #19 from psiroky/18-artifact-list

[#18] exception filters can now share list of missing/validated artifacts