text: Title: Save instance of results object on to disk
  Body: Sometimes at the end of long optimization runs, we would like to store the objects to disk for future use.
  
  One immediate use-case is https://github.com/scikit-optimize/scikit-optimize/issues/180
  
  For pickling, lambdas etc, I suggest we backport cloudpickle (https://github.com/apache/spark/blob/master/python/pyspark/cloudpickle.py) to `skopt.externals`, unless others have better ideas.
  