Git doesn't save empty dirs, so put a placeholder there that is
ignored when the test data is copied to its final location during
testing.

We now have full coverage of the main code.

With this solution, we can catch it cleanly in the standalone
nilmdb-server, and test the error paths in our normal test suite.

This is hard and finicky to test, so there's unfortunately not a clear
way to get 100% test coverage in cherrypy_start.  However, that
function is only used in the test suite and the standalone
nilmdb-server script, not in production (which goes through wsgi and
skips all this cherrypy server stuff entirely).

Note that argcomplete is also now required.

simplejson was there for python <= 2.6 compatibility.
It's not needed now, and installing it with pip can lead to segfaults
because pip is dumb.
(see e.g. https://github.com/simplejson/simplejson/issues/114)

We don't actually use nilmdb.utils.threadsafety.verify_proxy in the
main NilmDB code, but it's useful for finding errors.

It found an issue with __getattr__ in SerializerProxy which
(1) can't be avoided?
(2) is now commented in the code
(3) shouldn't matter in real use

No reason to push the content-type decision into httpclient

Previous commits went back and forth a bit on whether the various APIs
should use bytes or strings, but bytes appears to be a better answer,
because actual data in streams will always be 7-bit ASCII or raw
binary.  There's no reason to apply the performance penalty of
constantly converting between bytes and strings.

One drawback now is that lots of code now has to have "b" prefixes on
strings, especially in tests, which inflates this commit quite a bit.