Refactor validation classes

Add contest, candidate and result count validations for the 2002
general election.

To make the code more DRY, factor the counting methods into common
methods that handle both the primary and general election and move
these to the base ``MDElection`` class.

Move the election classes to a separate
``openelex.us.md.validate.election`` module.

Rename the validation helper classes ElectionYYYY[General|Primary]
instead of [General|Primary]YYYYElection to make the base class
names clearer.

Make the validation helper functions into methods of the
``MDElection`` class.

Addresses https://github.com/openelections/core/issues/36