Make .jsx the default extension

[hoppula]

I was debugging why my node.js app took several seconds to boot and finally found the culprit to be that having:

    require('node-jsx').install();

as first line of the app made all the subsequent require calls to go through React.transform.
My app starts now almost immediately after changing my React components to use .jsx extension and memory usage is 1/3 of original :)

README could possibly warn about this or .jsx could be set as default extension to prevent similar cases.

[Josh-a-e]

bump - I think this is a good idea.