Merge pull request #530 from aarondav/cleanup. Closes #530.

Remove explicit conversion to PairRDDFunctions in cogroup()

As SparkContext._ is already imported, using the implicit conversion appears to make the code much cleaner. Perhaps there was some sinister reason for doing the conversion explicitly, however.

Author: Aaron Davidson <aaron@databricks.com>

== Merge branch commits ==

commit aa4a63f1bfd5b5178fe67364dd7ce4d84c357996
Author: Aaron Davidson <aaron@databricks.com>
Date:   Sun Feb 2 23:48:04 2014 -0800

    Remove explicit conversion to PairRDDFunctions in cogroup()

    As SparkContext._ is already imported, using the implicit conversion
    appears to make the code much cleaner. Perhaps there was some sinister
    reason for doing the converion explicitly, however.