Add tests to sdist tarballs

[dvzrv]

Hi! When trying to package this project for Arch Linux I was attempting to run tests.
Unfortunately this is not possible, as sdist tarballs on pypi.org do not contain the test files and here on github only version 0.1 is available (see #90).

It would be very awesome if you could include the test files in the sdist tarball! :)

[dvzrv]

It seems that merging #25 and doing a successive release would fix this