Misc cleanups.

- The ``bobo.Application`` constructor now accepts objects as well as
  strings for the bobo options. This makes application definition from
  Python a bit cleaner.

- A new ``bobo_handle_exceptions`` options makes it easy to tell bobo
  not to catch application exceptions.  This is helpful is you're
  using WSGI middleware to handle exceptions.

- The object provided to ``bobo_errors`` option can now provide a
  subset of error handlers.