argument passed to liability must be Employee object

ArgumentError raised otherwise.  Test passes.  May not need to raise
error, however, since not getting user input.
changed Employee.new attributes to match other spec files
condensed Employee tests for attributes into single test
created test csv file for testing parse_data method
changed test Liability objects to be generated thru TaxCalulator.liability method; added a 2nd employee who is owed $
changed argument name to reflect amount_to_pay to gov't - was unclear before who was owed
TaxCalc liability method now explicitly returns liability_obj

Passed test before, but slightly clearer this way
changed employee obj names to reflect whether they get refund

Also moved comment
added test for employee_w_refund to make sure amount_owed is 0 if receive refund