allowed multiple select in membership app
updated invoices for membership applications
fix error for undeclared fields
merged with master
Merge branch 'refs/heads/master' into 443_multi_select_memberships

Conflicts:
	tendenci/addons/memberships/views.py
Merge branch 'master' into 443_multi_select_memberships. And added fixes
Merge branch 'master' into 443_multi_select_memberships. And updated migration file
Single invoice for multi-memberships
Merge branch 'refs/heads/443_multi_select_memberships'

Conflicts:
	tendenci/addons/memberships/views.py