Support custom default router

Add a new parameter to `express` function to provide a custom `Router`
constructor.

Example:

```js
var PromisedRouter = require('router-as-promised');
var app = express(PromisedRouter);
```

**Rationale**

This patch make it easier to experiment with different router implementations, as it enables developers (express users) to swap out the default Router implementation and replace it with their own.

 - As promises are coming to ES6, there is an initiative to get promise support into express (see #2259). The last recommendation was to create a new router implementation supporting promises ([promise-kernel](https://www.npmjs.org/package/promise-kernel)).

 - In LoopBack, StrongLoop's API framework based on express, we are experimenting with a different way of registering middleware (https://github.com/strongloop/loopback/pull/757). Our current solution involves overriding the default router used by express.

**Discussion points**

An existing workaround is to copy and edit `app.lazyrouter` to call a different Router constructor. This is very brittle as the copied version must be manually synchronized with any changes made in express.

Adding the first parameter to `express()` may be seen as controversial. Frankly, I don't really care about the API. I am happy to rework the PR to a different API as long as it allows me to change the Router ctor used by `app.lazyrouter`.

Example:

```js
var app = expres();
app.Router = MyCustomRouterCtor;

// perhaps a different name would be better?
app.routerFactory = MyCustomRouterCtor;
```

@dougwilson @jonathanong Thoughts?