model_utils: limit dependency to `model`.

Also update tools importing `model_utils`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: limit dependency to `model`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: clean output_dir option selection

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: extract NitdocContext from Nitdoc generation

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: Nitdoc standalone

Make `nitdoc` able to generate documentation only with a Model.

This will enable other clients than the actual `ModelBuilder` based one like from DB connector.

Pull-Request: #564
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>