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"
        ]
      }
    }
  }
}
```
I'm not sure about this being a class.

I feel just having two method taking a store as parameter and handling the skipping logic higher in the stack would make more sense.

I usually prefer my helpers to be functions rather than classes. It is more maintainable and easier to test. That's because functions are more self contained and have no implicit dependencies who're building code bloat (unlike classes who're all about implicit dependencies).

If you disagree, I'm okay to discuss it further.