Include .coveragerc in MANIFEST.in

I'm not sure what to do about samples/.  I'd like to keep them out of
the source tarball, but I'd also like to use them as a suite for
regression tests.  Conflicted.
Clarify that the arguments to compare.py can be in any order

(I'd forgotten that this script existed, and then forgotten how to use
it.  Good thing I've the habit of writing docstrings!)
Start tests for error handling

Adds a dependency on mock.