Delete note about History API.
Merge pull request #28 from mvano/HistoryNote

Delete note about History API
Remove pushRegistrationId from PushMessage. Fixes issue #32.
Merge pull request #34 from mvano/RemoveIdFromPushEvent

Remove pushRegistrationId from PushMessage. Fixes issue #32.
Use an updated image for the sequence diagram. Fixes issue #29.
Run tidy and add alt attribute.
Merge pull request #35 from mvano/SequenceDiagram

Updated image of sequence diagram (issue #29)
Cleaner steps for register, unregister, getRegistration.
Use specific errors. Resolve unregister with PushRegistration.
Run tidy.
Address Peter's comments.
Run tidy.