Implement toJson

[doug-wade]

2016-05-30T04:22:54.628Z - error: [react-server.core.renderMiddleware] Error rendering element Unknown message=toJSON() from superagent's API isn't implemented yet., stack=Error: toJSON() from superagent's API isn't implemented yet.
    at Request.(anonymous function) [as toJSON] (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react-server/target/server/ReactServerAgent/Request.js:39:10)
    at Object.stringify (native)
    at new StationList (station-list.js:22:34)
    at [object Object].ReactCompositeComponentMixin.mountComponent (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/ReactCompositeComponent.js:148:18)
    at [object Object].wrapper [as mountComponent] (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/ReactPerf.js:66:21)
    at Object.ReactReconciler.mountComponent (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/ReactReconciler.js:37:35)
    at [object Object].ReactCompositeComponentMixin.mountComponent (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/ReactCompositeComponent.js:225:34)
    at [object Object].wrapper [as mountComponent] (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/ReactPerf.js:66:21)
    at /Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/ReactServerRendering.js:42:38
    at ReactServerRenderingTransaction.Mixin.perform (/Users/doug.wade/code/react-server/packages/react-server-examples/bike-share/node_modules/react/lib/Transaction.js:136:20)