Add configurable ClassLoader for readers

Joe T. reported a bug where the default avro ClassLoader doesn't match the ClassLoader for the jar where his specific class definitions are:

Thread.currentThread().getContextClassLoader(); // ==> java.net.URLClassLoader@11c5ed81 <-- can load his avro-specific classes
SpecificData.get().getClassLoader().toString(); // ==> sun.misc.Launcher$AppClassLoader@111f2041 <-- avro instantiates GenericData.Record (can't find specific classes)
this.getClass().getClassLoader(); // ==> java.net.URLClassLoader@11c5ed81

Joe is using hadoop jar to run the reader command. I've suggested he separate the classes from his main jar and add them using the HADOOP_CLASSPATH env variable as a work-around.

CDK needs to allow the user to set the ClassLoader avro uses to find the specific classes, rather than relying on the default instance.