Use esprima-fb from NPM by default.

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