Add missing Capability enum values

[logiclrd]

When I tried to use this library today, I encountered errors trying to authenticate. After bashing my head against the wall trying to figure out what I was doing on for some time, I finally determined that the cause was actually nothing I was doing wrong. The JSON returned by Backblaze in response to successful authentication includes a list of Capability codes, and Backblaze have evidently added a couple of new ones, and the code blows up if it encounters Capability codes it doesn't recognize.

This PR adds new Capability enum values for the new codes returned by Backblaze:

• ReadBucketNotifications
• WriteBucketNotifications

I wonder also if there is some way to make the current structure of the code resilient to future changes of this type...