Count the number of successful assertions.

This does not work yet with the Runtime Loader script, which
seems to count each assertion twice. Also, using failIf() may
result in a wrong count.

We can only (easily) count successful assertions because a failure
results in an exception and the subsequent assertions are
not executed.
Fix wrong assertion count in should() and shouldnt().
Initialize assertions property to 0.

This is not strictly necessary, but it's better
to be explicit.
Count failIf() assertions.
Add tests.
Update TODO.