add caching

[dshemetov]

• add a cache argument to EpidataCall object
  • internally, if USE_EPIDATPY_CACHE env var is set to true, turn cache on
  • overwrite with whatever is passed in to the EpidataCall object
• if cache is on, cache result to XDG_CACHE_HOME (you can use appdirs user_cache_dir)
• if caching is on and the user is downloading recent data, print a warning about the cache becoming stale
• add cache timeout setting that will consider a cache stale after a certain amount of time