Load exception handler classes upon method lookup.

Since exception handlers need to check if the thrown exception is a subclass of
their handler class, their handler classes must be loaded in the handler's
classloader. We could conceivably do this at the time when the handler is
actually being evaluated against a thrown exception, but HotSpot seems to do it
at method lookup time, and that seemed easier to implement Doppio.

Note that I did not change all the method_lookup calls to fall back to
resolve_method in case of failure. Given that all the tests pass, I think we
should be okay. However, we really should get a compiler to generate this ugly
code boilerplate for us. Moreover, it would be able to safely bypass the async
fallback if the method has no handlers at all.

The small snippet in #176 now works correctly, but JRuby 1.7.3 still fails to
load its REPL, exiting quietly instead.