Improve parsing of voteview files, including a number of date formats.
Merge pull request #91 from GPHemsley/voteview

Improve parsing of voteview files, including a number of date formats.
Improve caching of YAML files. Simplify creation of new legislator maps and create person-congresses and congress-persons maps. Mark get_govtrack_person_id() and UnmatchedIdentifer as deprecated in favor of get_person_id().
Keep track of president's BioGuide ID, if it's available.
Add documentation comments for new utils functions.
Merge pull request #92 from GPHemsley/improve_cache

Improve caching of YAML files, and other fun stuff.
Merge pull request #93 from GPHemsley/voteview

voteview: Keep track of president's BioGuide ID, if it's available.