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()`.
Add javadoc with @deprecated here. Like this:

/**
 * Sends a password reset email to an email/password user.
 * @param {string} user
 * @deprecated
 */