Don't use PORT environment variable for grunt-express-server

This came out of setting up `ember-app-kit` for [FreshBooks](http://www.freshbooks.com/)' CI and builds infrastructure and colliding with another project's port.

Setting the environment variables for port configurations feels a bit odd and is obscure to locate. In addition, `PORT` is a very common environment variable that may collide with others.

By defining it explicitly in package.json, the user is able to quickly and easily change the port and check it into source control.

I explicitly left `process.env.PORT` alone in `tasks/options/testem.js` as I'm unsure if that's specific/relevant to grunt-express-server or not. It should probably be defined as `testemPort` in `package.json` anyhow. Thoughts?