Sketch of Arkansas datasource

This is a checkpoint commit for the Arkansas datasource.

There's still a lot of work to do, particularly around handling the
files that are zipped and the portal-generated files.

Addresses https://github.com/openelections/core/issues/124
Merge branch 'dev' into 124-ar_datasource
Populate AR jurisdiction mappings

Add counties to us/ar/mappings/ar.csv

Addresses https://github.com/openelections/core/issues/124
Merge branch 'dev' into 124-ar_datasource
Custom loader to extract zipped Arkansas results

This a checkpoint commit for implementing the Arkansas datasource
and loader that adds support for extracting the results from a
single zipped archive.

Currently only results for elections in 2000 are supported.

The datasource generates a mapping for each result file (including
ones archived in zip files) and adds a field to the mapping with
the archived file name.

For the general election, it's we can programatically generate the
filenames from the county names.  For the special elections, just
look things up from url_paths.csv

Addresses https://github.com/openelections/core/issues/124
Handle 2001 elections in Arkansas datasource

Addresses https://github.com/openelections/core/issues/124
Handle Arkansas reporting portal results

Update fetcher to get results from the the Arkansas Secretary of
State's election results reporting portal
(http://www.sos.arkansas.gov/electionresults/index.php).

We have to scrape the form in order to get the contest IDs to
include in the request parameters.

Addresses https://github.com/openelections/core/issues/124
Mappings for Clarity results

Produce mappings for results (2012+) that are produced by the
Clarity system.

Addresses https://github.com/openelections/core/issues/124
Clean up URL path construction

Clean up methods at build URL paths for the Clarity-based result
files.

Also write a header row in the per-election url_paths CSV files.

Addresses https://github.com/openelections/core/issues/124
Use XML for Clarity-based Arkansas results

I think the XML data is actually going to be the easiest to parse.

Addresses https://github.com/openelections/core/issues/124
Delete zip files after unpacking

Precinct-level results from the Clarity system all have the same
filename, so we need to delete the file after extracting it so
the file will be re-fetched for the next county.

Addresses https://github.com/openelections/core/issues/124
Don't make url_paths entry for county with no precinct results

Addresses https://github.com/openelections/core/issues/124
Merge branch 'dev' into 124-ar_datasource
Use generated Arkansas url_paths to repo

These are generated by scraping the Clarity system for certain
elections since, as far as I can tell, there isn't a way to
predict the identifiers in the URLs for the county-level election
summary pages and results.

Though I wanted to generate these from code to document and be
able to replicate my process, I think it makes sense to include
the output in the repo to save others the cost/time of rescraping
the information in their instances.

Addresses https://github.com/openelections/core/issues/124