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'"
Fixes markup on README
make virtus work in classes, too, by removing useless includes and attr_accessors.
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.
moving the :as section up as many people seem to "overread" it.
fix deserialization in JSON::hash, the original incoming hash was modified which should not be the case. thanks to @elvanja for bugging me https://groups.google.com/forum/#!topic/roar-talk/tDYxKTe1G6s