Fix STI+polymorphic for Group <-> GroupMember

app/controllers/groups/group_members_controller.rb

app/controllers/projects/team_members_controller.rb

app/models/members/group_member.rb

app/models/members/project_member.rb

app/views/groups/group_members/_group_member.html.haml

app/views/groups/group_members/update.js.haml

app/views/groups/members.html.haml

config/routes.rb

db/migrate/20140914145549_migrate_to_new_members_model.rb