Add support for inherited param handlers

This is a first cut at inherited param handlers. Likely this comes with corner cases, as I'm not an express internals expert, but it appears to work in straightforward cases. An additional parameter for `Router` is introduced to control this behavior:

```javascript
var router = express.Router({ inheritParamHandlers: true });

app.param('user', function (req, res, next, id) {
  User.findOne(id, function (err, user) {
    if (err) return next(err);
    req.params.user = user;
    next();
  });
});

router.param('photo', function (req, res, next, id) {
  Photo.findOne(id, function (err, photo) {
    if (err) return next(err);
    req.params.photo = photo;
    next();
  });
});

router.post('/update-tags/:photo', function (req, res) {
  // Error: contrived example
  req.params.user.update(req.params.photo, req.body, function (err) {
    if (err) return res.status(500).json({ status: 'error', error: err.message });
    return res.status(200).json({ status: 'updated' });
  });
});

app.use('/users/:user', router);
```

I've made it separate from `mergeParams`, as this may cause unexpected behavior for users if enabled via that option. However, it does enable `mergeParams` implicitly if used. A case could be made to consolidate the two into a single option which always does both as `mergeParams` was only publicly documented recently so perhaps the exposure is small.

The parameter handlers are kept in the `Router` but now follow the request throughout the `_router.handle()` path. This *should* allow different behavior if a router is attached to different apps or paths.

This would address issue #2151.
This is the problem; we don't want to really be adding a `req._paramHandlers` property to the request. To solve this, I have been working on an inter-router communication channel to avoid doing this. The main reason is adding a secret property to `req` does not allow for other people to make alternative router implementations without using secret express router channels.