Allow debug: option to be specified as (integer) port number.

I needed to be able to specify a custom debugging port while using grunt-express-server. This patch enables this functionality.

In addition to `debug: true` and `debug: false`, this patch will take an integer as a port number to start the node debugger on. All of these are valid examples:

```javascript
    express: {
      options: {
        port: process.env.PORT || 9000
      },
      dev: {
        options: {
          script: 'server/app.js',
          debug: 5959
        }
      },
```

```javascript
    express: {
      options: {
        port: process.env.PORT || 9000
      },
      dev: {
        options: {
          script: 'server/app.js',
          debug: false
        }
      },
```

```javascript
    express: {
      options: {
        port: process.env.PORT || 9000
      },
      dev: {
        options: {
          script: 'server/app.js',
          debug: true
        }
      },
```