Add `$update` action, make `$save` use `PUT` HTTP method when id field is set.
Add `$update` action, make `$save` use `PUT` HTTP method when id field is set.

For $save, this assumes that `ModelSerializer` is being used and supplying the primary key of objects as the 'id' field. If a different field name is used, the developer would have to override the save action's method_if_field_has_value setting.

Fixes blacklocus/angular-django-rest-resource#10.

sorenh/angular-django-rest-resource,pombredanne/angular-django-rest-resource,pombredanne/angular-django-rest-resource,sorenh/angular-django-rest-resource,mscienski/angular-django-rest-resource