fix(client): Check the user's old password before attempting a change password.

* Add 'checkPassword' to fxa-client.js
* Check the user's pasword before checking whether the passwords are the same.

fixes #1118

@zaach and @vladikoff - wanna check it out?
Probably, yes. Going to check. Did we miss this when doing the conversion earlier? If there is a reason, I'll make note of it.