This is in preparation for open sourcing React Native for Android.
Also add hyphens to URLs for consistency. This can break people's
browser bookmarks but it's better to be consistent and we can
redirect to docs: https://github.com/facebook/react-native/issues/2137
Test plan: Ran the website locally using `cd website; npm start`
and checked all pages render correctly.
http://i.imgur.com/RrPNgRr.png
Will update "Getting Started", "Tutorial", "Debugging" and
"Testing" separately.
