Added attribute scaffolding for models

For use as:
sails generate model User name:string birthday:date age:integer

I didn't account for mapping shorthand (like "int" or "str") to full versions.

This generates the nested model structure (instead of just using the type as a string) as that seems more forward-compatible (with defaultValue, etc).

Let me know if there are any problems or if I also need to update tests or adjust anything for conventions or completeness.