Fix sourcemaps, templating logic, and updates for React 0.15.5

• @0x80 noticed that source maps weren't working in Chrome in development and asked about it. That, combined with a serendipitous tweet by @gaearon, led to #812 where we swapped out using the eval source-maps technique for cheap-module-source-maps.
• @donysukardi got us React 16 ready by removing our uses of React.createClass and the built-in proptypes #809
• @danperkins fixed a logic bug in our router generation where nested templates weren't being tied to their parent templates correctly #808
• @KyleAMathews replaced the old default starter with a much simpler one to make it simpler to understand Gatsby when new plus easier to use the starter as a base for building new sites #806