Retry limits, custom permanent error codes, filesAdded event

Resumable.js is really awesome, thank you so much for creating it!

I made a few customizations to it to make it easier to work with in my project that may also benefit others as well.

1.  Added the ability to specify the maximum number of retries for a chunk before it results in a permanent failure.  I also added a chunk retry time to optionally slow down the rate at which the chunk gets retried.

2.  Added filesAdded event that contains an array of all the new files added instead of having to listen to each fileAdded event.

3.  Moved the permanent error codes into a configurable array so that I could add things like 401 to the list.