Added some options for model fill/create including role based fillable rules

**These may still need some tweaks and work, so mainly looking for feedback.**

## Multiple fillable roles
This is a suggestion to improve fillable to make it easier to assign multiple values for fillable. This would be useful if a site had different roles, for example if an admin can edit things a user cannot. The goal is to also maintain backwards compatibility with the current way to define fillable. This is how it would be used.

```
<?php
// In model
class User extends Eloquent 
{
    protected $fillable = [
        'default' =>  ['name', 'email'],
        'admin'   => ['name', 'email', 'role']
    ];
}
 
// When creating a model you can now do the following to control which "fillable" is used for mass assignment protection
User::create(Input::all(), 'default');
User::create(Input::all(), 'admin');
User::fill(Input::all(), 'admin');
 
// The following uses the default key, by default.
User::create(Input::all());
```

## Without Protection Option

I've also added another option called `without_protection` that will disable mass assignment protection for a single call to create, fill, etc. This would be simpler than calling guard and unguard over and over.

```
<?php
class User extends Eloquent 
{
    protected $fillable = ['name', 'email'];
}

// The following would ignore mass assignment protection and work properly. 
User::create(['name' => 'Ryan', 'email' => 'email@example.com', 'age' => 25]);