Run with Cloudera CDH 4.X

This mainly requires the changes below:

- Switch to using TaskAttemptContextImpl instead of TaskAttemptContext, in MyAvroMultipleOutputs, which is now an interface instead of a class that can be instantiated

- Use Avro 1.7.3, which fixes AVRO-1170 (that prevented avro-mapred from being used with hadoop2 API's); also cajole Ivy to use the Maven classifiers necessary for doing that

- Make sure that classes from included Avro libraries are loaded before Hadoop runtime with the mapreduce.task.classpath.user.precedence job config property; this seems like a good idea even w/o CDH compatibility woes.
Updates for CDH 4.5
fix issue with types of fields in Avro