Add back shade plugin

Generate "fat" JAR separate from the main JAR, which can then be used by
the assembly plugin. Simplify pom a bit.

pom.xml

@@ -82,7 +82,6 @@
     </resources>   
     <plugins>
       <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>
         <configuration>
           <archive>
@@ -105,9 +104,7 @@
           </descriptors>
         </configuration>
       </plugin>
-      <!-- Optional shade with BCEL
       <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-shade-plugin</artifactId>
         <executions>
           <execution>
@@ -116,12 +113,13 @@
               <goal>shade</goal>
             </goals>
             <configuration>
+              <shadedArtifactAttached>true</shadedArtifactAttached>
+              <minimizeJar>true</minimizeJar>
               <createDependencyReducedPom>false</createDependencyReducedPom>
             </configuration>
           </execution>
         </executions>
       </plugin>
-      -->
     </plugins>
   </build>