Updated API signatures of user management methods

@katowulf - Please review and merge. This has two changes:
1. Updates the user management methods to take either separate string parameters (as it currently does) or a credentials object (as Firebase 1.1.x+ does).
2. Deprecates `$sendPasswordResetEmail()` in favor of `$resetPassword()`.
Please add a proper JavaDoc to this showing that the second argument is optional, otherwise IDEs will spit a warning whenever this is accessed. This would work:

```
/**
 * Creates a new email/password user. Note that this function only creates the user, if you
 * wish to log in as the newly created user, call $authWithPassword() after the promise for
 * this method has been resolved.
 *
 * @param {Object|string} email
 * @param {string} [password]
 */
```