Can now pass optional keys to omit when cloning a model

Can now do:
```
model.clone({omit: "id"})
```
or
```
model.clone({omit: ["name", "firstname"]})
```
There might be other ways to do that but I find it comfortable.

I chose `omit` instead of `except` so it stays consitent with underscore method's name.
Having an `only` option could also be useful.