Tell PyPI long_description is Markdown

Rendering of the README on PyPI is broken because it expects RST by default and Markdown is being uploaded. This fixes it.

setup.py

@@ -53,6 +53,7 @@
         "Programming Language :: Python :: 3.6",
     ],
     long_description=long_description,
+    long_description_content_type='text/markdown',
     tests_require=('nose', 'nosexcover'),
     test_suite='nose.collector'
 )