Add 2014 primary election results to datasource

* Add 2014 county-level PDF to url_paths.csv
* DRY up logic that creates mappings for precinct-level Excel results for elections in 2006 onward
* Update _precinct_xls_metadata to use .xlsx extension for elections starting in 2014
* Add jurisdiction column to url_paths.csv and use it in the canonical filename.  This was needed because there are per-county, precinct-level results for some special elections

Addresses #118