v0.13.3 - Documentation updates, new sound dictionary, and better error reporting

This release updates documentation (and the payload builder) to keep up with recent upstream changes. The major changes in this release are:

• Updated documentation links to point at Apple's new UserNotifications framework docs instead of the now-deprecated Local and Remote Notification Programming Guide.
• Added support for the new sound dictionary to ApnsPayloadBuilder in order to support newly-introduced "critical notifications"
• Improved error handling to increase the chances that we'll have a more specific error message than "stream closed before a reply was received"

For a complete list of changes, please see the v0.13.3 milestone.