Maryland 2004 contest, candidate and result count validation

Add validators for contest, candidate and result counts for the
2004 primary and general elections.

Move some common functionality shared between elections into
mixin classes.

2004's general election includes results for the Baltimore City
Comptroller office, but this isn't identified by a separate office
string.  Add a validator to detect this and a transform to remove
the records.

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