Aliased :from to :as in accessor property

When creating a document from an object the DSL language was confusing, so this change allows you to do both:

    property :title, :from => :display_title

and:

    property :title, :as => :display_title

It would read more like: Display the property 'title' in the document as
'display_title'"

Let me know if there are other cases you would like to add test coverage, or if there is a better way to implement the feature. I didn't really get too deep into the code since I found the easy way to get this done.