payment-unit-tests

• v0.1: initial payment unit test [payment constructor (importTsv), link/unlink methods (payment-offer), update]
• v0.1.1: changes in tests, additional tests, remove comments
• v0.1.2: refactor/improve tests [assert exception] ~add missing exceptions
• v0.1.3: fix failing tests (more tests TBD)
• v0.1.4: fix assert checks, remove comments