CDH4 support (Hadoop 2.0.0 + Pig 0.10.0)

This commit is based on https://github.com/kevinweil/elephant-bird/pull/287 (sorry not sure how to add commit to someone else's pull request).

It enables EB to support CDH4, which uses the following versions:
- Hadoop:  org.apache.hadoop/2.0.0-cdh4.2.0
- Pig:  org.apache.pig/0.10.0-cdh4.2.0

It passes all unit tests but I think more work should be done to maintain backward compatibility with previous versions of Hadoop.  I'm open to suggestions on how to go about doing that.