Ignore foreign ipn

[hoverlover]

This PR includes code that will ignore an IPN message if the invoice doesn't correspond with a Spree order. Sometimes, PayPal users will receive funds outside of Spree. With the current codebase, this will cause a 500 error because the order can't be found. The code I've included will simply return a 200 response for the IPN request if the invoice param doesn't match the format of the Spree order number.

[tomash]

i don't like the params[:invoice].index condition

i don't like it because it's tied to default spree order numbering system, and doesn't guarantee that it's an actual order in the db.

what i did in my own store (haven't expected it to be useful to other people) :

private
  def return_for_inexisting_orders
    @order = Order.find_by_number(params[:invoice])
    if(@order.nil? || params[:invoice].nil?)
      render :text => 'not found', :status => 200, :layout => false
    end
  end

other than that, good job! i'd prefer feature-related commits and testing-environment-related ones to be in separate pull requests, though.