Add custom audience example and add default ad_account to config

Summary:
Adds an example for custom audiences that demonstrates
create, delete and listing.

Test Plan: create, delete list
cleaning up fields for custome audinece object to be compatible to latest doc

Summary: As it says above

Test Plan: 1/ ca = CustomAudience(<ID>) 2/ ca.remote_read(fields=[input all fields here]) 3/ check the ca object fields