Add a React Native example

[flowersinthesand]

Thanks to cettia/cettia-javascript-client#24 to be included in 1.1.0, it's possible to use cettia object in React Native environment. Let's write an example that runs on React Native.

After Cettia JavaScript Client 1.1.0 is released,

• Add a 'React Native' section to README.md.
• Update the getting started guide.
• Remove the note if Cettia JavaScript Client 1.1.0 is released

[flowersinthesand]

Not to reinvent the wheel,

• Load example-web's resources into WebView
• Load the cettia object and open a socket outside WebView
• Make them communicate