Added generate_test_file class method to FeedBuilder

Hi Denis!  I've had to extend this code a bit to make it meet current client needs.  One thing I found myself doing regularly was going into the console to generate an XML dump for us to look over.  I went ahead and built a class method and rake task to do just that.  It leverages all of your existing functionality, just a slightly different sequence of calls.

Hope you can use this!
Daniel