Expose esprima-fb's "Syntax"

This PR exposes `require('esprima-fb').Syntax` from `jstransform/src/utils` and updates the references to it throughout.

Motivation: I find myself doubly including `esprima-fb` because of the weird version numbers it uses and my need to reach `Syntax`. The most visible example of this is with [envify](https://github.com/hughsk/envify) (used by React), which requires `jstransform` and `esprima-fb`. Since the `jstransform` version can float, you end up in a situation where `jstransform` uses X version of `esprima-fb`, but `envify` is using Y version solely for the `Syntax` object.

Hopefully if this gets merged, I'll make a pull request to simplify [envify](https://github.com/hughsk/envify) and maybe we can bump envify in [React's next release](https://github.com/facebook/react/issues/2137).