Fix issue with default scope clashing with deletion.

Fixes #37

In certain edge case scenarios default_scopes can interfere with the SQL Paranoia generates to issue a record hide.  Both `update_column` and `update_attribute` use default scopes. Take the following edge case scenario:

```ruby
class User < ActiveRecord::Base
  default_scope :include => :login, :order => "logins.first_name" 
  belongs_to :login, :class_name => "OleCore::Login"
  acts_as_paranoid
end
```

In this case Paranoia throws the following SQL error when trying to save: `ActiveRecord::StatementInvalid: PG::Error: ERROR:  missing FROM-clause entry for table "logins"`

My particular case is a very edge case scenario, but I can see the default scoping being a broader problem.

By adding unscoped to the deletion query default scope issues are avoided. In this case since we're simply setting an attribute on an already found model, adding unscoped shouldn't impact the functionality of Paranoia at all.