End Bacon.fromPromise() handler chains with .done() if present

Some major promise packages, such as https://github.com/kriskowal/q
will silently eat exceptions happening in promise handlers unless the
promise chain is ended with .done(). This patch ensures
Bacon.fromPromise ends such promise chains.

See https://github.com/kriskowal/q/wiki/Coming-from-jQuery#exception-handling

Fixes #501