rename User class to avoid namespace collisions

Background
----

When `Plivo` is used as a mixin (as in all of the examples provided), it pollutes the `User` namespace, which means examples like the following will fail:

```ruby
class DailyMessageWorker
  include Plivo
   def perform
     plivo.send_message({'dst' => user.phone_number,
         #other params    
     })
   end

  def user
    User.where(hour_to_message: Time.now.hour).last
  end

  def plivo
    @plivo ||= RestAPI.new(PLIVO_CONFIG[:auth_id], PLIVO_CONFIG[:auth_token])
  end
end
```

Plivo will probably be used in the context of background workers. A model likely to have phone numbers is the `User` model, which is common to nearly all Rails applications. Hence namespace collisions seem highly likely.

The change herein addresses this issue.