Add the List#unique_list_id per changelog

https://labs.aweber.com/docs/reference/1.0#list_entry now shows
the unique list id.  This is useful for building forms out of data
from API calls.

This attribute entered at API version 1.0.21 per the changlog at
https://labs.aweber.com/docs/changelog
Add unique_list_id to test fixture.

I don't think the current test reads this but it is good for
completeness.
Merge pull request #30 from Kajabi/bem-add_unique_list_id

Add the List#unique_list_id per changelog