v0.13.6 - Payload properties for notification summaries

This is a minor release that adds support for the summary-arg and summary-arg-count payload fields, which allow notifications that get "stacked" on the receiving device to have custom summary strings (e.g. they can be used to change "4 more notifications" to, say, "4 more photos from Matt").

For a complete list of changes, please see the v0.13.6 milestone.