Added linkContacts method (#180)

* Added linkContacts method

When viewing the Infusionsoft developer docs for linking contacts support, there seems to be no PHP support, while XML-RPC and Python do have support (see https://developer.infusionsoft.com/docs/xml-rpc/#contact-link-contacts). I noticed this while working on a project for a client when attempting to link a child contact with a parent contact. After researching, I found this method that I could add to the ContactService class (see this outdated post: https://community.infusionsoft.com/t/how-can-i-link-contacts-via-the-php-isdk/2935).

* Changed minor formatting error

src/Infusionsoft/Api/ContactService.php

@@ -152,5 +152,16 @@ public function update($contactId, $data)
  {
  return $this->client->request('ContactService.update', $contactId, $data);
  }
+
+ /**
+ * @param integer $contactId1
+ * @param integer $contactId2
+ * @param integer $linkId
+ * @return integer
+ */
+ public function linkContacts($contactId1, $contactId2, $linkId)
+ {
+ return $this->client->request("ContactService.linkContacts", $contactId1, $contactId2, $linkId);
+ }
 
-}
+}