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?
Ah okay. And we don't use `/account/keys` on its own, rather we sign in with the `keys` option and allow for unverified signin, so this should be okay.