first draft of the extension generator

### Description

Enter the desired values to create a Yii2 extension ready for usage with composer.
There are currently three files generated: composer.json, README.md and AutoloadExample.php.

**This is not final yet.** I need some input.

### Questions

Q: Should we enforce the `yii2-` prefix for the package name? 
I'd say yes. Will keep a consistent naming across extensions. Can still be changed manually.
How can I prepend yii-2 in the input field or render a form field without label?

Q: Which values should be available for `type`, do we need more? 

Q: How to handle the array input for keywords?
We could add the most popular tags from Yii1.
I'd like to do this input with a Select2 widget, but is it available already (in the core).

Q: What do you think about autogenerating the namespace, is the JavaScript implementation sufficient?

Q: Should title also be autogenerated?
ucFirst(packageName)

Q: Where should the extension be generated?

Q: What do you think about the testing explanation.


Validation and Prefix and Namespace rendering have to be improved, but I wanted to clear the above points first.

![bildschirmfoto 2014-02-13 um 04 21 33](https://f.cloud.github.com/assets/649031/2156506/ffdf26d2-945d-11e3-87a4-73bfa707842d.png)
![bildschirmfoto 2014-02-13 um 04 21 45](https://f.cloud.github.com/assets/649031/2156505/ffbadd04-945d-11e3-963c-9550c813627a.png)