Add early abort option to Tester:run()

The program will finish when any of the tests fails or generate an error.
Early abort activated via command line argument

--early-abort is now a command line argument that allows to stop
the execution of a series of tests when one of them fails or
generate an error.
Small fix

Remove unused parameter.
Remove duplicated computation of number of tests
Merge pull request #6 from akfidjeland/early_abort

Early abort option