Rename listener methods

[ColinEberhardt]

Currently we have the following:

intentListener(intent: String, handler: (context: Context) => void): Listener;
contextListener(handler: (context: Context) => void): Listener;

In keeping with web conventions (e.g. the DOM addEventListener method), and the standard approach of using 'verb phrases' for method names, I'd propose adding an add prefix to each:

addIntentListener(intent: String, handler: (context: Context) => void): Listener;
addContextListener(handler: (context: Context) => void): Listener;

[nkolba]

@ColinEberhardt can you make a PR for this?