exclude _testdoubleload.so from binary eggs

[zooko]

The eggs resulting from python setup.py bdist_egg have a copy of _testdoubleload.so in them. That file is absolutely necessary for the buildbot-operated tests of #44, so we can't just get rid of it, but it should be excluded from binary distributions.

(I like binary distributions to be able to run unit tests, but running the test double load test is not so important for them to do that they need to carry around that .so file.)

That file is created by python setup.py build (and the other commands that automatically build) but only when the --test-double-load option is passed.

Hopefully we can exclude it from distributions just by editing MANIFEST.in​

[zooko]

The double load tester .so appears to be absent from bdist_egg results according to my manual test. I don't know what patch changed this.