Regression test for pseudo-hierachy via iterator delimiter and prefix.

Fixes #372.
Add a comment explaining testing deeper than two levels of pseudo-hierarchy.

Incorporates feedback from @dhermes:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/376#discussion_r20826236
Merge pull request #376 from tseaver/372-pseudo_hierarchy_regression_test

Add regression test for pseudo-hierachy via iterator delimiter and prefix.