Adding failing unique-constraint tests
better key checking in set() to avoid saving "null" key
adding utils.is_number()
making sure all test docs are removed at end of client tests
adding uniqueness constraint checks on User.create() for username and email.
reluctantly exposing User._make_key()
cleaning up orphaned reference docs on failure
Notes
User.destroy(): removing reference docs
User.update() - deleting old reference docs
adding User._destroy_reference_docs()
minor cleanup
User.update() - updating reference docs
failing test for dangling references when making simultaneous updates to a user
test rename
refactoring test - assert_no_dangling_references()
test name fixes
testing that we can set an email
unique constraints working, no dangling references
minor refactoring