Make AccumulatorParam an abstract base class.
Remove use of abc.ABCMeta due to cloudpickle issue.

cloudpickle runs into issues while pickling subclasses of AccumulatorParam,
which may be related to this Python issue:

    http://bugs.python.org/issue7689

This seems hard to fix and the ABCMeta wasn't necessary, so I removed it.
Merge pull request #396 from JoshRosen/spark-653

Make PySpark AccumulatorParam an abstract base class