Allow id and email identifiers to be updated when cio_id specified

[richdawe-cio]

This is an alternative solution to #78 . It's an alternative to the solution implemented in #102 .

This PR updates the identify operation to use the cio_id to identify customers, if it's present. This will allow users of the client library to update customers using the existing identify call by passing in the cio_id, and whatever attributes they want. This may include the id (external ID) or email, but those would not be required.

Changing the id and email for a customer with cio_id of "f000000d" would then look like this:

$customerio.identify(
  :cio_id => "f000000d",
  :id => 1005,
  :email => "bob.fullname@example.com"
)