Enhance models structure

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I think the models should be divided into two classes, e.g., UUser.php and 
User.php. The prefixed models would reside in a _base folder. This would be 
great for many reasons:

1. It allows users to place their code in separate file which makes it easier 
to update the extension.

2. It makes easier to combine extensions since you can place more classes 
between User and UUser. 

In my case, my class User extends BaseUser (from giix extension) which, in 
turn, extends UUser.

Original issue reported on code.google.com by m...@saulovallory.com on 24 Sep 2011 at 11:04