Add a Site facade class to standardize and simplify library usage

[zstumgoren]

Create a court_scraper.site.Site class that dynamically looks up and instantiates the appropriate site class based on placed_id. This will free end users from having to dig around for the appropriate site class for the respective court platforms.

• Refactor Wicourts to:
  • use standard Site name
  • automatically set download_dir (from base SeleniumSite class) if not provided in search or search_by_date methods
  • dynamically look up captcha_key if not provided to init
• Refactor OdysseySite to
  • use standard Site name
  • automatically set download_dir (from base SeleniumSite class) if not provided via init