Change date format to ISO 8601

This came up on the mailing list.

yyyy-mm-dd (ISO 8601) is the internationally accepted format for numeric
dates. This commit changes all BIPs to use that instead of dd-mm-yyyy.
It also updates BIP 0001 to prescribe the new format.

bip-0011.mediawiki

@@ -4,8 +4,8 @@
   Author: Gavin Andresen <gavinandresen@gmail.com>
   Status: Accepted
   Type: Standards Track
-  Created: 18-10-2011
-  Post-History: 02-10-2011
+  Created: 2011-10-18
+  Post-History: 2011-10-02
 </pre>
 
 ==Abstract==