manage contact_group_link model

. create table contact_group_links (rename the unused
manytomany_contacts_groups table)
. add contact_group_linksController (PHP)
[Fix] while creating a contact_group_link
add contact_group_link_ids in GET/POST/PUT responses
Merge pull request #1 from Kcnarf/relationship-persistancy(2)

Relationship persistancy(2)