Flexible handling of callback for auth methods

[mattheworiordan]

See https://jsbin.ably.io/ukinas/2/edit. It fails because the callback provided is the second argument instead of the third, however for other methods like requestToken it is allowed as the second argument, see

ably-js/common/lib/client/auth.js

Lines 210 to 218 in 1cf5108

	/* shuffle and normalise arguments as necessary */
	if(typeof(tokenParams) == 'function' && !callback) {
	callback = tokenParams;
	authOptions = tokenParams = null;
	}
	else if(typeof(authOptions) == 'function' && !callback) {
	callback = authOptions;
	authOptions = null;
	}

.

I propose for all Auth methods createTokenRequest and authorise, we provide the same consistent flexibility that is common in JS.

@SimonWoolf mind looking at this whilst doing your other bits?