Add validation for sub-contest votes and unique candidates

Add validators that sum sub-contest votes and compare them against
known contest totals from the Maryland Board of Elections website.

I only wrote one validator per data file layout/loader class, but
I implemented some helper functions that should make it easy to
validate the totals for other elections.

Also add a validator that checks that there is a unique set of
candidates for each contest.

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