Add set_poll_interval and get_poll_interval methods to the base PollingSensor
class.

This way sensor can call those methods to retrieve and adjust the poll interval
if necessary.
Update changelog.
Merge branch 'master' into allow_polling_sensors_to_retrieve_and_set_poll_interval

Conflicts:
	CHANGELOG.rst
Merge pull request #931 from StackStorm/allow_polling_sensors_to_retrieve_and_set_poll_interval

Allow polling sensors to retrieve and set poll interval