Commits on Dec 23, 2015

1. Support for optional attributes …

   This commit introduces support for optional attributes. It avoids
   silently assigning the value `nil` to attributes that are not
   present in the original input, but are attributes of the validator
   object
   
   Consider the following, similar to the example in the README:
   
   ```ruby
   post.inspect
   
   class UpdatePost < Scrivener
     attr_accessor :title
     attr_accessor :author
   
     def validate
       if assert_present :title
         assert_length :title, 3..150
       end
     end
   end
   
   input = UpdatePost.new({ :title => "A Changed Title" })
   input.valid?
   ```
   
   This is correct. No assertions for `:author` are made, so no errors
   are expected. When using the output of `#attributes` however, data
   loss is very easy to occur.
   
   ```ruby
   post.update_attributes(input.attributes)
   post.inspect
   ```
   
   That's because `#attributes` assigns `nil` for "missing" attributes.
   
   ```ruby
   input.attributes
   ```
   
   This commit changes `#attributes` to not assign `nil` to "missing"
   attributes, so they become optional.
   
   ```ruby
   input.attributes
   ```
   
   It's still possible to make assertions on optional attributes:
   
   ```ruby
   class UpdatePost < Scrivener
     attr_accessor :title
     attr_accessor :author
   
     def validate
       if assert_present :title
         assert_length :title, 3..150
       end
   
       if author
         assert_match :author, /[A-Z]\w+ [A-Z]\w+/
       end
     end
   end
   ```
   
   The change is subtle, but existing code might expect missing
   attributes to be `nil. If it is to be merged, I think it's wise to
   bump the version to 2.0.

   Jip van Reijsen committed Dec 23, 2015
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