The content of holocron example post is improved

This commit changes the text of holocron.mdown file, adding info about holocron
features and its basic usage.

Fixed #16
This is an image of a holocron:
![jedi_holocron](https://cloud.githubusercontent.com/assets/921482/4044630/6765a0f8-2d22-11e4-9715-266612ba6c6c.png)
Do you think we should include it?