Option for users to change their registered e-mail

Blueprint 'change-email' is implemented by sending an activation link
to verify the new email id. The activation link contains a code that
has an expiry period. Added corresponding tokens to the API method.

* regservice/keystone/views.py
  + Added email_activation_code and email_activation_code_time tokens
to 'get_user' API method.

Implements: blueprint change-email