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'"
adding a test for JSON::hash :songs, :extend => .., :class => ..
Merge branch 'alias-from-to-as' of git://github.com/csexton/representable into ass
releasing 1.3.1.