F/496 multiple user schema support

You can now have multiple types of users/user schemas per app.

* user schema is now determined by StackMob.User.extend({ schemaName: ... }), not StackMob['userSchema']
* loginField and passwordField are now defined on the User Schema (defaults to old StackMob['loginField'] and StackMob['passwordField'])
* specs for loginField and passwordField