Feature/s3 emr target

This pull request offers a quick way for testing if EMR output on S3 is present and finished.  Given an S3 location, it will check:

1.  That the location exists
2.  That the _SUCCESS file exists within the location, signaling that the EMR output is complete.

This is accomplished via the `S3EmrTarget`.  The `S3EmrTask` external task is also offered as a convenience.

Confusing Terminology Alert
-------------------------------------

The `_SUCCESS` file is a Hadoop construct.  This isn't checking that an EMR "step" completed, but rather that a Hadoop job completed.  I opted to call it the `S3EmrTarget` instead of the `S3HadoopTarget` to signal that its EMR-friendly.  I'm open to a better naming convention,