When chaining methods to purge by surrogate key, get_service API call is executed every time

[kigster]

We needed to purge objects by a surrogate key, and had the code shown in the image below (red). We were instantiating Fastly.new object, then calling get_service(), and then chaining purge_by_key(key). This usage ends up calling an API to get service every time for each purge, and we ended up exceeding rate limits for the API.

The solution is shown in green, and should probably be documented somewhere. Perhaps another class Purge should take care of executing these calls efficiently, given that all keys and api_keys are known ahead of time.

[drbrain]

@kigster how does #51 look to you?