Update the build to allow setting source/target for the java compiler.

This is to allow 1.6 and 1.7 support in the same build.

Add a "-Djava.target=1.6" to the ant build parameters and the system builds a java 6 compatible version of JDBC4 in to the jars, and correctly deploys it to the nexus maven repostiory. By default this uses java.target set to the java version of the build system (java 1.7/JDBC4.1).