Added support for dropping column lists, and warning if name of column list to be created already exists. If exists, prompts user to run DROP COLUMN LIST... first.
Added support for dropping row lists, and warning if name of row list to be created already exists. If exists, prompts user to ruN DORP ROW LIST... first.
Converted returned dict from engine.show_row_lists to have 'column_labels' to conform to other engine functions
Added client testing for DROP ROW LIST, and parser testing for both DROP COLUMN LIST and DROP ROW LIST
Merge branch 'master' into list-overwrite
Resolved merge conflicts with master
Merge pull request #41 from mit-probabilistic-computing-project/list-overwrite

List overwrite