Add groupby support on dataframes to SparkLing pandas.

Add groupby support on dataframes to SparkLing pandas.
minor changes:
-Add a helper method to make a distributed dataframe from a local dataframe (this results in types being reguessed so it is less than ideal)
-add back in a run-tests script that can yell if SPARK_HOME isn't set (whereas directly using nose just resulted in no output).
-Add inital code coverage, although we might want to disable it since it only reports on things which run locally and most of our stuff runs remotetly.
-Remove some dead code from utils. 
-update unittest library to unittest2 so we can use expected failure in py2.6