Add some test.
Correct handling of JS exceptions in handler callbacks.

Javascript exceptions in handler callbacks were not handled in the C++ code
properly. Instead of breaking of the handler entirely and returning the
Javascript error from the apply() call, the excpetions were ignored. Only the
last exception in one of the handler callbacks was thrown from the apply().
Fixes #22.
Robustify handlers.

Because of the bug fixed in the last commit, the 'count++' lines were never
called leading to confusing error reports. By incrementing the count right
were we check it, something like this can't happen any more.
Simplify tests: Getting rid of done() calls.