Added better associations and UsersUnit model

Before this change we had:

    2.0.0p195 :029 > customer.member_list
      Membership Load (0.3ms)  SELECT "memberships".* FROM "memberships" WHERE "memberships"."memberable_id" = 1 AND "memberships"."memberable_type" = 'Customer'
      Customer Load (0.2ms)  SELECT "customers".* FROM "customers" WHERE "customers"."id" = 2 LIMIT 1
      Customer Load (0.2ms)  SELECT "customers".* FROM "customers" WHERE "customers"."id" = 3 LIMIT 1
      Customer Load (0.2ms)  SELECT "customers".* FROM "customers" WHERE "customers"."id" = 4 LIMIT 1
     => [#<Customer id: 2, name: "ciao", created_at: "2013-10-25 19:32:26", updated_at: "2013-10-25 19:32:26">, 
         #<Customer id: 3, name: "otra", created_at: "2013-10-25 19:37:05", updated_at: "2013-10-25 19:37:05">, 
         #<Customer id: 4, name: "otra mas", created_at: "2013-10-25 19:37:19", updated_at: "2013-10-25 19:37:19">]

but now...

    2.0.0p195 :002 > customer.customers
      Customer Load (0.1ms)  SELECT "customers".* FROM "customers" INNER JOIN "memberships" ON "customers"."id" = "memberships"."member_id" WHERE "memberships"."memberable_id" = 1 AND "memberships"."memberable_type" = 'Customer' AND "memberships"."member_type" = 'Customer'
     => [#<Customer id: 2, name: "ciao", created_at: "2013-10-25 19:32:26", updated_at: "2013-10-25 19:32:26">, 
         #<Customer id: 3, name: "otra", created_at: "2013-10-25 19:37:05", updated_at: "2013-10-25 19:37:05">, 
         #<Customer id: 4, name: "otra mas", created_at: "2013-10-25 19:37:19", updated_at: "2013-10-25 19:37:19">] 

Furthermore now we could create new relationships just calling:

    customer.customers << new_customer