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router: add async route handler support

While the async/await feature is still highly experimental and not
official, it does have preliminary support in tools like babel. There
are even blog posts [1] about how to use the async/await feature with
express routes.

However, these posts along with issue #2789 and #2788 show that using
async/await is still a bit of a manual process and not clear. This
change inspects the return value of a route handler to see if it is
potentially a pomise. If the return value appears to be a promise then
we attach to the error handler of the promise via .catch()

Now a user who is using babel can write the following without needing
any wrap functions as most examples to date require.

```js
app.get('/', async (req, res, next) => {
    let user = await User.findById(); // assuming .findById() returns a primise
    let org = await Org.findById();
    res.json({
        user: user,
        org: org,
    });
});
```

[1] https://strongloop.com/strongblog/async-error-handling-expressjs-es7-promises-generators/

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