Categories for contacts

Since evolution allows the use of categories, macaco-contacts should provide the same functionality. The functionality provided should sallow the following:

* Add a list of categories to a contact. Separate the categories by using commas.
* List all contacts within a category.
* If all contacts are removed from a category, the category should not longer be shown.
* Delete a category. The system should modify all those contact with such category and will remove it.
* Rename categories. The system should change that category from all the categories in the contacts that have them.
* Add more than one contact to a category. (Drag and drop)