Cache attribute table

[jonathan-marsan]

For GetAttribute... functions, need to find a way to cache the table so the table isn't fetched every time one of the functions is used.

[yosemsweet]

The way I approached this for KissReport and KissReports is to create an empty env within the S3 object, and then in the read function I first check to see if we already have the appropriate data in the cache using readCache from utilities.R and if not then we'll load the data and write it to cache using writeCache.

You might also consider adding a refresh_cache flag to the read call, as the url for properties and events never changes so our cache keys shouldn't change. By providing an optional flag we can support new attributes being added during a session.