Bulk create support

[krzysztofgromadzki]

Related issue: #204

This implementation just invokes pre_save_polymorphic method from PolymorphicModel for each saving object just before django's bulk_create . Usually this method is used in save method implementation in PolymorphicModel. Particullary pre_save_polymorphic method checks that foreign key to ContentType is set. If not, sets it.
Other possible solution (I didn't test it) is moving pre_save_polymorphic call to __init__ in PolymorphicModel and removed this call from save method. In this case overidded bulk_create won't exist

[vdboor]

Cool! I think this is quite a nice solution! Django's bulk_create also performs a list(objs) so there is no performance issue here. While moving the pre_save_polymorphic logic to PolymorphicModel.__init__() could be possible, you'll have to be careful with overriding models. This solution avoids such traps! :-)