Added locking of SecureString, wasn't thread safe

CryptoExchange.Net/ExtensionMethods.cs

@@ -30,6 +30,8 @@ namespace CryptoExchange.Net
         }
 
         public static string GetString(this SecureString source)
+        {
+            lock (source)
             {
                 string result = null;
                 int length = source.Length;
@@ -55,3 +57,4 @@ namespace CryptoExchange.Net
             }
         }
     }
+}