Add ``MANIFEST.in`` file.

Fixes #8
Do not install tests

`python2.7 setup.py install` needlessly installs `test` directory as
`/usr/lib/python2.7/site-packages/test`.

Inclusion of `test` directory in source tarball is already provided by
`recursive-include test *.html *.md *.py` line in `MANIFEST.in`.

Credit: https://github.com/Alir3z4/html2text/issues/36