added  change password method with criteria and response model..

https://careerbuilder.mingle.thoughtworks.com/projects/engagement__new_/cards/137
As a jobseeker I want to change my password.
Change password : Added a new method change_pwd which works based on response,criteria model.
Currently reset password is using change_password method so i could not modify the same if so it will break the functionality of reset password.
Added rspec for criteria and for the client user