Send deprecation warnings in a standard way

Add addDeprecationWarning to `helpers.js`, taking a kind, deprecated thing and its replacement.  Warnings are stored and emitted to the warn level of the logger at the end of an appium session.

cc @imurchie

(Attempts to make this a seperate PR from https://github.com/appium/appium/pull/2034 only partly successful;  This should only be merged after that)