Checkpoint commit for Iowa Loader

Implement the scaffolding for the Iowa loader including the base
classes for both the preprocessed and Excel results.

Update the mappings to skip loading of some precinct-level results
from a single statewide file.  This appears to be the same information
as the per-county precinct files, and doesn't have candidate names
or district numbers.

Still to do is to finish implementing loading of post-2010,
precinct-level results.

Addresses https://github.com/openelections/core/issues/203
Loader for Iowa precinct-level results from 2010 onward

Addresses https://github.com/openelections/core/issues/203
Merge branch 'dev' into ia
Fix mispelling of Van Buren county in a transform
Create separate RawResults for absentee, provisional votes

In the Iowa loader, create separate RawResult records for
absentee and provisional votes.

Also, change the call to _xls_file_handle() to use the
_xls_file_path property since it was renamed.

Addresses https://github.com/openelections/core/issues/211
Add reporting level to mappings

A couple of rows in Iowa's url_paths.csv lacked a value in the
reporting_level column.  This reflects the changes in the
openelex-data-ia package made in
https://github.com/openelections/openelections-data-ia/commit/86c980a043413831d2a58d493ee6b92ef4d4cb1a
Fix download of 2010 general Linn County precinct results

The 2010 general election precinct-level result file for Linn
County inexplicably has an "xlsx" file extension while the other
files have an "xls" extension.

Addresses https://github.com/openelections/core/issues/203
First pass at Iowa 2010 General Election result loader

Also factor out logic to pick the loader for a particular mapping
because there are going to have to be lots of loaders.
Handle alternate column layout for 2010 Iowa results

Some 2010 General Election precinct-level results in Iowa
have the vote breakdowns in columns of the same row rather
than as separate rows.  Handle both cases.

Addresses #203
Merge branch 'dev' into ia
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
Merge branch 'dev' into ia
Loader clas for 2012 results

Implement loader class for 2012 Iowa general and primary
elections.

Stub out loader classes for 2014.

Update LoadResults._get_loader() to route to these loader classes.

Addresses https://github.com/openelections/core/issues/203
s/polling/election_day/

Use the more easily understood slug 'election_day' to denote
election day votes in Iowa, even though 'polling' is used in
one particular election.

Add 'election_day' to openelex.models.VOTES_TYPE_CHOICES.

See https://github.com/openelections/core/issues/216
Merge branch 'dev' into ia
Iowa 2014 primary loader

Addresses https://github.com/openelections/core/issues/203
Support 2014 Iowa State House special election

Properly set name field for mappings based on values in url_paths.csv

Use name in mappings to detect county if there is no county column
in the preprocessed results CSV.

Addresses https://github.com/openelections/core/issues/203
Merge branch 'dev' into ia
Merge branch 'dev' into ia