Merge pull request #248 from zmaril/master

Clarification on the effect of 'Has Missing'
some more predicate tests
better explanation of missing data
Don't encourage users to run a hanging command.

`nosetests` hangs on some machines, while `coverage run -m nose -I
canonical_test` doesn't seem to. For onboarding new developers, let's
not tell them to use commands that might not work.