Support for Heartbeat-type of methods

[fohrloop]

Splitting this work from https://github.com/fohrloop/wakepy/issues/69 into its own ticket.

Motivation

• Some methods might need a "heartbeat"; some call (executable, dbus, ...) done periodically.

Task

• Create a Hearbeat class which runs the Method.heartbeat() in separate thread. It uses the Callers to process calls.

As there are not yet any Methods which utilize a heartbeat, this can be postponed until some release after the 0.8.0.