Modified pom to be able to do a maven release to maven central.

The most important change (which might break something outside of the core maven build) is:

-    <appendAssemblyId>false</appendAssemblyId>
+    <appendAssemblyId>true</appendAssemblyId>

in the maven-assembly-plugin section.

This causes both jar with dependencies and jar without dependencies to be created.
Only the jar without dependencies (and the sources and javadoc jars) should be uploaded
to maven central.