Use SS_Cache to cache expensive API calls

[madmatt]

Caching needs to be carefully thought through, given we're connecting to an external API, this may be slow to access, but we need to ensure we're as up to date as possible, given the external API could change at any time (users are expected to be using both the FreshBooks interface as well as these classes to modify their FreshBooks account, so data could change at any time).

Some potential low-hanging fruit for caching (perhaps an hour cache lifetime?):
* category.list, category.get (any other category.* method should invalidate or update the cache)
* client.list, client.get (any other client.* method should invalidate or update the cache)
* estimate.list, estimate.get (any other estimate.* method should invalidate or update the cache)

Same goes for all the other get/list methods, they're pretty similar. Cache lifetimes will need to be carefully decided.