Emit fail event

Allow to define a 'failAfter' option ([I've done a pull-request](https://github.com/MathieuTurcotte/node-backoff/pull/11) on ```backoff``` to add this feature) and dispatch a 'fail' event if that happens. This way, you can define a limit of unsuccessful tries instead of keep trying infinitely.
Setting failAfter option directly instead of on backoff module
Merge pull request #8 from piranna/patch-1

Emit fail event