Add long_description_content_type to setup.py

The new PyPI code discards long_description when it's not valid ReStructuredText.  Luckily it also supports Markdown, but you have to [specify the content type](https://packaging.python.org/tutorials/packaging-projects/#creating-setup-py) explicitly in that case.

(IIRC for this to work correctly you also have to use sufficiently recent versions of setuptools and twine to upload releases.)

Should fix #303.

setup.py

@@ -18,6 +18,7 @@
       version=versioneer.get_version(),
       description="Securely transfer data between computers",
       long_description=open('README.md', 'rU').read(),
+      long_description_content_type='text/markdown',
       author="Brian Warner",
       author_email="warner-magic-wormhole@lothar.com",
       license="MIT",