Make possible to create new user role

[jgrossi]

This fixes #349. Basically:

$role = (new RoleManager())->from('editor')
    ->create('foo', [
        'edit_pages' => false,
        'edit_others_pages' => false,
    ]);

It creates a new foo role based on editor overriding some capabilities, in this case: edit_pages and edit_others_pages.

This is going to be merged on 2.5 but if everything is ok it's gonna be merged into 2.4, 2.3, 2.2 and 2.1 as well.

[Stevemoretz]

@jgrossi May I ask why you didn't merge this?

[Elvinca]

@jgrossi I would like to know aswell...