Merge pull request #10 from jwhitlock/1063621_add_changesets

bug 1063621 - add changesets
bug 1089375 - Switch to drf_cached_instances

drf_cached_reads moved to separate repo:

https://github.com/jwhitlock/drf-cached-instances
fix bug 1089375 - Refactor duplicated cache code

Move code for loading primary keys of related items into new function,
call from the loader and serializers.