add category field to APS struct
Don't serialize Alert if all fields are blank

ref #5

For Passbook and Newsstand notifications.
update byte payload test (no longer contains alert)
Update example
Merge pull request #11 from alytvynov/master

Add category field to APS struct
Update docs to note that `Feedback.Receive` is not an indefinite connection

* Closes #6
A different take on empty APS
Note to update isZero when adding Alert fields
notification: shuffle code around
Merge branch 'master' into empty_alert

Conflicts:
	notification.go
Merge branch 'empty_alert' into safari

Conflicts:
	notification.go