Reverse context from liquid_methods

This patch adds the option to add the context from a liquid drop back into the original instance:

app/models/page.rb:

    class Page
      liquid_methods :render, :reverse_context => :context
      def render
        Liquid::Template.parse(content).render(self.context)
      end
    end

This way, `{{ page.render }}` will render the Page's content in the context provided to the drop.