Cleans up class as well as updates readme (#32)

Author: amosharrafa

README.md

@@ -341,6 +341,25 @@ ad_set.destroy
 
 ___
 
+### [Ad Set Activities](https://developers.facebook.com/docs/marketing-api/reference/ad-activity) (Fetch)
+
+You interact with activities via an ad set:
+```ruby
+ad_set = account.ad_sets(effective_status: nil).first
+```
+
+Fetch all activities in last 24 hours for an ad set:
+```ruby
+activities = ad_set.activities
+```
+
+Fetch all activities in last 48 hours for an ad set:
+```ruby
+activities = ad_set.activities(since: 2.days.ago)
+```
+
+___
+
 ### [Ads](https://developers.facebook.com/docs/marketing-api/reference/adgroup) (Fetch, Find, Create, Update, Destroy)
 
 You interact with ads via an ad set:

lib/facebook_ads/ad_set.rb

@@ -110,7 +110,7 @@ def ad_insights(range: Date.today..Date.today, level: nil, breakdowns: [], field
     end
 
     # Retrieves activities for ad set (in last 24 hours by default)
-    def activities(since = 1.day.ago)
+    def activities(since: 1.day.ago)
       AdSetActivity.get("/#{id}/activities", objectify: true).select { |activity| activity['event_time'] > since }
     end
   end

lib/facebook_ads/ad_set_activity.rb

@@ -2,9 +2,6 @@
 
 module FacebookAds
   # https://developers.facebook.com/docs/marketing-api/reference/ad-activity/
-  # curl -G
-  #      -d 'fields=actor_id,actor_name,event_time,event_type,extra_data,translated_event_type'
-  #      -d 'access_token=<access_token>' https://graph.facebook.com/v3.1/<ad_set_id>/activities
   class AdSetActivity < Base
     FIELDS = %w[
       actor_id