Remove CORS headers from fullResponse (can conflict with restify.CORS)

The new `CORS` middleware has great support for cross-origin requests, including:

- defining a list of allowed origins
- defining a list of custom exposed headers

The `fullResponse` middleware also does some of this, but unfortunately:

- not as configurable
- sometimes conflicts with choices made by the CORS middleware

For example, it will not override `Access-Control-Allow-Origin` if it's already set ([see code](https://github.com/mcavage/node-restify/blob/master/lib/plugins/full_response.js#L35)), which is good. However the CORS module might decide **not** to add the header, for example if the request doesn't match the allowed origins ([see code](https://github.com/mcavage/node-restify/blob/master/lib/plugins/cors.js#L94)). In this case setting it as `*` later is probably not the right behaviour.

There might be ways to arrange the middlewares & configs to avoid these clashes, but I think having a single full-featured `CORS` middleware is preferable to having CORS support added in a few places. Thoughts?