Fixed Unit Tests under latest API version

There were a number of issues running the unit tests under the latest version API version:
* Date-specific checks fail if testing machine isn't exactly in sync with Stripe machine (adjusted test to allow upto 1 min difference)
* API 2013-02-11 changed behaviour to throw exception when paying an invoice instead of setting paid to false.
* Compiler warnings due to Machine.Specifications style (suppressed)