Added get_indexed_instance method

When a signal handler is run on a model that is the parent in a multi
table inheritance relationship, we do not have access to all of the
fields we need to properly reindex the object.

This commit adds a method that the developer can use to specify how to
get the specific type of a particular instance which we can use to
reindex it.
Merge branch 'search/index-correct-object' of https://github.com/kaedroho/wagtail into kaedroho-search/index-correct-object
Merge branch 'kaedroho-search/index-correct-object'