allow for multi word labels and added label test
Updated _column_order_by and estimate_columns to return values of column functions in addition to column names
Added function to return a basic description of each function by which column lists are ordered
Refined labels of function value columns for estimate columns
Adjust handling of estimate columns with no order by to pass tests
Updated tests for estimate columns
Renamed function_descriptions to order_func_descriptions to be clearer
Merge branch 'dev' into column_list_values
Add numsamples argument to all functions
Merge branch 'dev' of github.com:mit-probabilistic-computing-project/BayesDB into dev

Conflicts:
	bayesdb/engine.py
	bayesdb/tests/test_parser.py