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 is going to be shared by every instances. My comment was just about making it private inside each instance, prefixing by a underscore `_`