Add 'production' option

Hello,

I ran into issue where I do not want to install project devDependencies (usefull for build or release script). 
I created a new "production" option named according to the ```bower install``` command one.
So now you can install bower's packages without the dev dependencies

For example

```js
grunt.initConfig({
  bower: {
    install: {
      options: {
        copy: false
      }
    },
    install_prod: {
      options: {
        copy: false,
        production: true
      }
    }
  }
});
```