IE8 support for JSXTransformer

This depends on a pending PR (https://github.com/facebook/esprima/pull/4) for esprima/fb-harmony, this is not harmful without that PR, but IE8 support won't be functional without it.

The esprima PR is simply changing ```default: def``` to ```'default': def```

(Very sorry, my initial extra PR was for recast where I led myself to believe that I had found the problem, the real fix is the following for esprima/fb-harmony https://github.com/facebook/esprima/pull/4).