Remove over, under and straight party vote type ids

RawResult.votes_types should only reflect types of votes that can be
broken down on a per-candidate basis.

Over and under votes aren't associated with a candidate.  Instead
they function as pseudo-candidates.

Addresses https://github.com/openelections/core/issues/211
Merge branch 'dev' into ia
Allow load.run task to load a single file

Add a --filename option to the load.run task that will load a
single specified filename.

Add BaseDatasource.mapping_for_file to return a single mapping
based on a standardized, generated filename.

Add tests for the new BaseDatasource methods.

Addresses https://github.com/openelections/core/issues/127
Merge branch 'dev' into ia
Handle hidden columns in Emmet, Van Buren results

The 2010 general election, precinct-level results for Emmet and
Van Buren counties in Iowa have hidden columns.  Hack around this.

:scream:

https://github.com/openelections/core/issues/203