Task to bake results metadata JSON

Add an invoke task, ``bake.results_status_json``, to bake a JSON
file describing available results for each election.  This will be
consumed by the results front end JavaScript.

Usage:

inv bake.results_status_json --state=md > ~/workspace/openelections.github.io/data/elections-md.json

Added ``base.publish.published_url()``, a utility function to
generate the publicly accessible URL for a results filename.

Fixed ``lib.standardized_filename`` so the state abbreviation bit
is always lowercased.

Refactored code that flattened elections fetched from the
datasource.

Fixed ``base.bake.reporting_levels_for_election`` to include
a state argument.

Addresses https://github.com/openelections/core/issues/164