[Nitro] Entity -> Model

[George Moschovitis]

Dear devs,

in the quest for consistency across the Nitro sub-frameworks I have
renamed Og::Entity to Og::Model.  Please pull the latest repo version
before sending further Og patches.

The rational is that model is used in Raw/Nitro and is more
understandable to newcomers coming from AR/MVC.

-g.

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