Allow to createUsers with email and username

First of all thanks for this great Package! :+1: 

Anyway it is not possible to create a new User with `username` **and** `email`.

Since this is part of the Meteor [Accounts](http://docs.meteor.com/#accounts_ui_config) package I suggest the following fix.

This allows to pass an Object to `Asteroid.prototype.createUser`.

**Example:**

	var ceres = new Asteroid('localhost:3000');
	ceres.createUser({
		email: 'max@example.de'
		username: 'Max',
		password: 'secret123',
		profile: {}
	}).then(...);

To be backwards compatible it is of course possible to use `Asteroid.prototype.createUser` in the old way.