Move middleware sources to `server/middleware`

The new location allows developer to use the following identifiers
when loading the middleware using the new declarative style:

    app.middlewareFromConfig(
      require('loopback/server/middleware/rest'),
      { phase: 'routes' });

    app.middlewareFromConfig(
      require('loopback/server/middleware/url-not-found'),
      { phase: 'final' });

Gruntfile.js

@@ -36,6 +36,9 @@ module.exports = function(grunt) {
       common: {
         src: ['common/**/*.js']
       },
+      server: {
+        src: ['server/**/*.js']
+      }
       // TODO tests don't pass yet
       // test: {
       //  src: ['test/**/*.js']
@@ -44,7 +47,8 @@ module.exports = function(grunt) {
     jscs: {
       gruntfile: 'Gruntfile.js',
       lib: ['lib/**/*.js'],
-      common: ['common/**/*.js']
+      common: ['common/**/*.js'],
+      server: ['server/**/*.js']
       // TODO(bajtos) - test/**/*.js
     },
     watch: {

lib/loopback.js

@@ -120,13 +120,16 @@ if (loopback.isServer) {
 
 if (loopback.isServer) {
   fs
-    .readdirSync(path.join(__dirname, 'middleware'))
+    .readdirSync(path.join(__dirname, '..', 'server', 'middleware'))
     .filter(function(file) {
       return file.match(/\.js$/);
     })
     .forEach(function(m) {
-      loopback[m.replace(/\.js$/, '')] = require('./middleware/' + m);
+      loopback[m.replace(/\.js$/, '')] = require('../server/middleware/' + m);
     });
+
+  loopback.urlNotFound = loopback['url-not-found'];
+  delete loopback['url-not-found'];
 }
 
 /*

server/middleware/context.js

@@ -1,4 +1,4 @@
-var loopback = require('../loopback');
+var loopback = require('../../lib/loopback');
 var juggler = require('loopback-datasource-juggler');
 var remoting = require('strong-remoting');
 var cls = require('continuation-local-storage');

server/middleware/rest.js

@@ -2,7 +2,7 @@
  * Module dependencies.
  */
 
-var loopback = require('../loopback');
+var loopback = require('../../lib/loopback');
 var async = require('async');
 
 /*!

server/middleware/token.js

@@ -2,7 +2,7 @@
  * Module dependencies.
  */
 
-var loopback = require('../loopback');
+var loopback = require('../../lib/loopback');
 var assert = require('assert');
 
 /*!