Document Model::Validate

Add documentation for Batman.Model::Validate, Batman.ValidationError and Batman.ErrorsSet. 

Includes a bonus commit with a line of plain text to fix the code rendering on [Model.load](http://batmanjs.org/docs/api/batman.model.html#class_function_load) (screenshot [before](https://f.cloud.github.com/assets/2231765/1423286/55215818-4008-11e3-81b9-8fd152e4c165.png), [after](https://f.cloud.github.com/assets/2231765/1423291/60fbee14-4008-11e3-9aaf-cd3085358f32.png))