Markdown readme

[janosh]

Convert the readme from rST to markdown:

pandoc --from rst --to gfm README.rst --output README.md

Any objections @shyuep? markdown is more readable, much more widely used and understood than rST so should allow more people to contribute updates/corrections.

[janosh]

This change allows us to have a single source of truth for GH and PyPI pages since setup.py currently contains basically a diverged copy of the rST readme as markdown.

[coveralls]

Coverage increased (+32.9%) to 32.908% when pulling 7eb3e16 on markdown-readme into 29b0552 on master.