Add timestamps macros for setting up timestamps

It will be in this way
```ruby
class Product
  include Mongoid::Document
  field :name, type: String
  field :description, type: String

  timestamps
  # timestamps :created
  # timestamps :updated
  # timestamps :short
  # timestamps created: :short
  # timestamps updated: :short
```

What do you think? 
If you will accept I will re-factor and add specs