Use vote_breakdowns and votes_type correctly

My original intent was to eliminate ``RawResult.vote_breakdowns`` and instead create separate ``RawResult`` records with different ``votes_type`` values.
Then we could aggregate these into ``Result.votes_breakdowns`` in a Transform.  It turns out that transform is really, really slow because for each result, you have one query to get all related RawResults that provide the breakdowns.

Using vote_breakdowns is a good, space efficient way to represent this.  We should keep using it and instead only use ``votes_type`` to disambiguate the case when the value in the ``votes`` field isn't the total because the source file only includes one kind of vote tally (frequently election day).

There should be one RawResult per candidate, per row in the source data.  In some cases, this means setting ``vote_breakdowns``.  In others, it means creating a separate ``RawResult`` for each type of vote and setting ``votes_type`` and aggregating these in a transform.  The goal should be to keep loaders simple and make ``RawResult`` instances roughly follow the layout of the source data.

Add ``votes_type`` to ``Result`` model.

Use ``None`` instead of an empty string for the value of ``votes_type`` when the value of ``votes`` are total votes.

Update Iowa loaders to use either ``votes_type`` or ``votes_breakdown`` when appropriate.  Fix setting of ``votes_type`` flag in ``PreprocessedResultsLoader._prep_precinct_result()``.  The ``votes_type`` field was being set to 'absentee_provisional' regardless of whether it was actually an absentee or provisional vote.

Set the ``vote_breakdowns`` field in ``ExcelPrecinct2010GeneralResultLoader`` if the breakdowns are in the same row.

Update ``ExcelPrecinct2010GeneralClintonResultLoader`` to produce a ``vote_breakdowns`` field.

In Maryland's ``MDLoader._prep_precinct_result()``, set ``votes_type`` to ``election_day`` instead of setting ``vote_breakdowns``.  There aren't any vote breakdowns, just no total votes.

Update NC loaders to set ``RawResult.votes_type`` when appropriate.

Addresses https://github.com/openelections/core/issues/211