Middleware phases - initial implementation

Modify the app and router implementation, so that the middleware is
executed in order defined by phases.

Predefined phases: 'init', 'routes', 'files', 'final'

Methods defined via `app.use`, `app.route` and friends are executed
as the first thing in 'routes' phase.

API usage:

    app.middleware('init', compression());
    app.middleware('before:init', serveFavicon());
    app.middleware('after:files', loopback.urlNotFound());
    app.middleware('after:final', errorHandler());

Middleware flavours:

    // regular handler
    function handler(req, res, next) {
      // do stuff
      next();
    }

    // error handler
    function errorHandler(err, req, res, next) {
      // handle error and/or call next
      next(err);
    }

See also:
 - The discussion in #757, especially my last comment
 - #739 (the work item)

**Out of scope - will be done later**

 - More predefined phases
 - API for adding custom phases
 - Draft of Confluence docs

/to @ritch @raymondfeng please review
/cc @fabien @superkhau