Adds support for multiple column (backwards compatible)

This PR allows defining multiple orderable columns ( fixes #23 ) like:

```
class Book
  include Mongoid::Document
  include Mongoid::Orderable

  orderable
  orderable column: serial, default: true
end
```

The code sets up a class_attribute **orderable_configurations** which is a hash of all the configurations and then generates helpers like:
```
@book.move_serial_to :top 
@book.next_serial_items
```

This code change is backwards compatible, so existing implementations will also work.
```
@book.move_to! 3
@book.next_item
```

In these case, the default configuration (default: true) or the first configuration is used.

Tested it for Mongoid 2, 3, 4 on a Rails 3 app - https://github.com/bharat311/mongoid_orderable_demo