Initial implementation of prove_it flag for tests.  If success is not pr...

Exception based handling of unit tests have always bothered me as by design they assume success.  This commit changes that in minitest so that the developer is notified that they haven't proven anything if they haven't asserted a condition.  My preference would be to enable this by default but that may prove to be too aggressive for some at this point in time.

This is a proposal for inclusion into minitest, it is not yet complete.  I have not added unit tests or updated the unit tests to use this flag.  If you feel it should be included in the core I'll follow up with more commits.  If not I'll look into building a plugin.