Pushbullet notification sensor (#7182)

* Added the pushbullet sensor component

* Updated requirements_all.txt and .coveragerc with the new sensor

* Updated acording to houndci-bots comments

* Some more changes

* Final change by the hound (?)

* Fixes from balloobs review and from houndci-bot

This changes the sensors information to only contain one attribute
as information, and the rest as device_state_attributes.

* Added leading space to comments

* Added docstrings, removed API_KEY from log, changed imports

* The hound is at it again

* Fix remaining issues

* Fix pylint issue