Drush web service: add drush-server option for runserver command

Hi there,

This needs work but thought I would submit for some feedback before proceeding much further.

My use case is that I am writing tests for a Drupal site using CasperJS. I want to be able to run some Drush commands in my setUp and tearDown methods (and possibly elsewhere). It seemed simple enough to provide an interface for doing so with the existing `drush runserver` command.

This PR extends `runserver` by adding a `drush-server` option. So you can run `drush rs --drush-server` and have a web interface at `http://127.0.0.1:8888`. You can then do, for example, `http://127.0.0.1:8888/@example.dev core-status --format=json`.

No consideration whatsoever is given to security. This is meant for local development purposes.

Some feedback that would be helpful:

1. Would this be better off in its own command?
2. Would this be better in a separate project from Drush?

If you are open to having this included in Drush, then let me know what you'd like to see added/changed.