Add extracted AccountRoster model

Uses [this
service](http://sparkplatform.com/docs/api_services/my_account#RetrieveAccountInfoByIdForRosters)

Author: bhornseth

lib/spark_api/models.rb

@@ -8,6 +8,7 @@
 
 require 'spark_api/models/account'
 require 'spark_api/models/account_report'
+require 'spark_api/models/account_roster'
 require 'spark_api/models/activity'
 require 'spark_api/models/connect_prefs'
 require 'spark_api/models/contact'

lib/spark_api/models/account_roster.rb

@@ -0,0 +1,9 @@
+module SparkApi
+  module Models
+    class AccountRoster < Account
+      def self.roster(account_id, arguments={})
+        collect(connection.get("/accounts/#{account_id}/roster", arguments)).first
+      end
+    end
+  end
+end