Adds create to list_membership

Copied from pardot#42, shout out to @polygonplanet

Author: ebinmore

lib/pardot/objects/list_memberships.rb

@@ -1,36 +1,41 @@
 module Pardot
   module Objects
     module ListMemberships
-
       def list_memberships
         @list_memberships ||= ListMemberships.new self
       end
 
       class ListMemberships
-
-        def initialize client
+        def initialize(client)
           @client = client
         end
 
-        def query params
-          result = get "/do/query", params, "result"
-          result["total_results"] = result["total_results"].to_i if result["total_results"]
+        def query(params)
+          result = get '/do/query', params, 'result'
+          result['total_results'] = result['total_results'].to_i if result['total_results']
           result
         end
 
-        def read_by_id id, params = {}
+        def read_by_id(id, params = {})
           get "/do/read/id/#{id}", params
         end
 
+        def create(list_id, prospect_id, params = {})
+          post "/do/create/list_id/#{list_id}/prospect_id/#{prospect_id}", params
+        end
+
         protected
 
-        def get path, params = {}, result = "listMembership"
-          response = @client.get "listMembership", path, params
+        def get(path, params = {}, result = 'listMembership')
+          response = @client.get 'listMembership', path, params
           result ? response[result] : response
         end
 
+        def post(path, params = {}, result = 'listMembership')
+          response = @client.post 'listMembership', path, params
+          result ? response[result] : response
+        end
       end
-
     end
   end
-end
+end