Create new API entity: ActivityContact

After merging tables activity_assignment, activity_target into activity_contact (see CRM-12274), it would be interesting to write a new entity ActivityContact.

This entity would facilitate the manipulation via API of the different participants of an activity, no matter if they are source, target or assigned contacts.

There is already a first version of the API entity (see attached ActivityContact.php) but we still need some tests before sending the Pull Request.

This topic has been discussed previously at:

• http://forum.civicrm.org/index.php/topic,32218.0.html
• http://forum.civicrm.org/index.php/topic,31718.msg135996.html#msg135996