Remove time unit and column from python and make time the first column

[heimir-sverrisson]

This PR addresses issue #8 and issue #9. The time_column and unit arguments have been removed from the TimeSeriesDataFrame python constructor. If the time column is not the first column it is moved to be the first column of the result. This behavior was already enforced in Scala and is now properly handled in Python including the code that caused issue #8.