Added string conversion for resources.

Author: tjlytle

src/Application/Application.php

@@ -113,4 +113,9 @@ public function jsonSerialize()
             'type' => 'voice' //currently the only type
         ];
     }
+
+    public function __toString()
+    {
+        return (string) $this->getId();
+    }
 }

src/Numbers/Number.php

@@ -178,4 +178,9 @@ function jsonSerialize()
 
         return $json;
     }
+
+    public function __toString()
+    {
+        return (string) $this->getId();
+    }
 }