Fix #133:  preserve PB namespace in Key.from_protobuf.

See #133.
I have a strong preference (see my blog entries [1][2]( for avoiding imports at module scope in test code, particular imports of the module / package under test.  Import failures in the module-under-test (MUT) should cause individual test cases to fail: they should never prevent those tests from being run. Depending on the testrunner, import problems may be much harder to distinguish at a glance than normal test failures.

[1] http://palladion.com/home/tseaver/obzervationz/2008/unit_testing_notes-20080724
[2] http://palladion.com/home/tseaver/obzervationz/2009/unit_testing_redux-20090802