store prompt answers in a global config file

In addition to this previous Pull Request https://github.com/yeoman/generator/pull/579.

This Pull Request, adds the ability to save the answers from a inquire prompt in a global config file `.yo-rc-global.json`. The answer will be set in the future as the default answer. 

To enable this feature just set `store: true` on each prompt which you want to save. The default value is set to `false`.
```
var prompts = [{
    type: 'input',
    name: 'githubUsername',
    message: 'What is your GitHub username',
    store: true
  },
  {
    type: 'list',
    name: 'licence',
    message: 'Choose a licence',
    choices: ['MIT', 'GPL', 'Apache'],
    store: true
  },{
    type: 'password',
    name: 'password',
    message: 'Your password'
    // Will not saved
  }
];

this.prompt(prompts, callback);
```

You can clear the global cache directly from your yo cli. Take a look at the related PullRequest https://github.com/yeoman/yo/pull/227

### Changes i did

The `storage.js` takes now a third (optional) argument `version` in his constructor. This is need to bind the answers to a specify generator version. 


The structure of `.yo-rc-global.json` looks like this 
```
{
  "generator-node-gulp": {
    "0.5.3": {
      "promptValues": {
        "githubUsername": "stefanbuck",
        "authorName": "Stefan Buck",
        "modules": [
          "jscsModule",
          "istanbulModule"
        ],
        "dependencies": [
          "lodash",
          "debug"
        ]
      }
    }
  }
}
```
This variable here will be kept in closure between generators instances. You should decouple the logic from the middle step object.

My guess is that you should repass the initial user prompt config to the `storeAnswer()` method.