Read default serializer options from the controller

[andrusha]

This allows you to do something like:

helper do
  def default_serializer_options
    {only: params[:only], except: params[:except]}
  end
end

And then on any route you could fetch only fields you actually need. There are many other use-cases of course, but this was my main motivation.

[andrusha]

Tests are failing because of #24. @sonxurxo could you take a look?

[jrhe]

Can you write some tests for this and also document it in the readme? Ready to merge otherwise.

[siong1987]

@jrhe i added the required spec on #30 and also updated the README. merge?