Introduce list functionality

[legovaer]

I'm currently working on a module called marketo_subscribe as we needed functionality for users to be able to subscribe to a list in Marketo with only their e-mail address (no body field).

As the functionality for Lists in Marketo was missing in this module, I created two new public methods to the API Client:

• addLeadsToList() => Adds a given array of Lead objects to a list using the addLeadsToList() method on the base client
• addLeadToListByEmail() => Creates a new lead for the given e-mail address and then calls addLeadsToList().

I've also added unit tests for both the functions.

[chx]

This has been committed on drupal.org.