* Significant refactor of task list in operations.py
Classes based on composite pattern replace tuples for
tasks.
* Addressing review comments for eland/operations.py
* Minor update to review fixes
* Minor fix for some better handling of non-aggregatable fields: https://github.com/elastic/eland/issues/71
* Test for non-aggrgatable value_counts
* Refactoring tasks/actions
* Removing debug and fixing doctest