[SPARK-7969] [SQL] Added a DataFrame.drop function that accepts a Column reference.

Added a `DataFrame.drop` function that accepts a `Column` reference rather than a `String`, and added associated unit tests.  Basically iterates through the `DataFrame` to find a column with an expression that is equivalent to that of the `Column` argument supplied to the function.

Author: Mike Dusenberry <dusenberrymw@gmail.com>

Closes #6585 from dusenberrymw/SPARK-7969_Drop_method_on_Dataframes_should_handle_Column and squashes the following commits:

514727a [Mike Dusenberry] Updating the @since tag of the drop(Column) function doc to reflect version 1.4.1 instead of 1.4.0.
2f1bb4e [Mike Dusenberry] Adding an additional assert statement to the 'drop column after join' unit test in order to make sure the correct column was indeed left over.
6bf7c0e [Mike Dusenberry] Minor code formatting change.
e583888 [Mike Dusenberry] Adding more Python doctests for the df.drop with column reference function to test joined datasets that have columns with the same name.
5f74401 [Mike Dusenberry] Updating DataFrame.drop with column reference function to use logicalPlan.output to prevent ambiguities resulting from columns with the same name. Also added associated unit tests for joined datasets with duplicate column names.
4b8bbe8 [Mike Dusenberry] Adding Python support for Dataframe.drop with a Column reference.
986129c [Mike Dusenberry] Added a DataFrame.drop function that accepts a Column reference rather than a String, and added associated unit tests.  Basically iterates through the DataFrame to find a column with an expression that is equivalent to one supplied to the function.

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