Added a new setting, HAYSTACK_IDENTIFIER_METHOD, which will allow a cust...

...om method to be provided for ``haystack.utils.get_identifier``.

Hey guys, I'm going to be needing this at work sometime soon so I thought I'd contribute it here first to get some feedback.  We're in a situation where we need the identifier to be globally unique, so this patch allows us to do that by providing a new method for ``haystack.utils.get_identifer``.

I owed Daniel a pull request anyway for that commit bit he gave me.

- Andrew