Attributes starting with an underscore are filtered out also. Renamed to purgeUnneeded to reflect new function.

```php
$user = new User();
$user->save(Input::all());
```

I use the above code alot when creating new users from a register form, for example. I ran into problems with this because it would include `_token` and `_method` in as attributes.  When saving to the database, it throws an error because those are not fields in the table.  I added some code to the `purgeRedundant()` method to filter these out as well.  I also renamed that method to `purgeUnneeded()` to reflect its expanded usage.