[changed] Transition#cancelReason => abortReason

Also, transition.abort() now accepts a reason argument that indicates
the reason for the abort. I anticipate that we'll need to add a NotFound
reason soon.
[added] onAbortedTransition, onActiveStateChange, onTransitionError Routes props

Also, removed Routes.handleAsyncError and Routes.handleCancelledTransition
Clean up test harness
[added] <DefaultRoute> component

Also, changed behavior of routes with no name, path, or children so
they act as default routes. <DefaultRoute> is essentially just sugar.

Fixes #164
Fixes #193
Fix failing specs