Use esprima-fb from NPM by default.

Note that you can still use whatever parser you want by calling `recast.parse` with `options.esprima` equal to any object with a compatible `.parse` method: `recast.parse(source, { esprima: require("your-esprima") })`.

I will merge this as soon as @zpao or @jeffmo publishes the latest version of `esprima-fb` to NPM.

Closes #105.
Ah yes, good point.