MANIFEST.in: reduce size of PyPi source distribution

I don't think people use the PyPi bundle to actually run tests or
compile documentation and so experiment with removing both from the
MANIFEST.in file. This reduces the size of the download from ~2MB (which
already lacked much of the documentation) down to ~200KB.

Certainly the common case of "pip install dtcwt" will build neither the
documentation or run the test suite. Although perhaps it should?
Merge pull request #54 from rjw57/make-pypi-smaller

MANIFEST.in: reduce size of PyPi source distribution