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.
Merge pull request #45 from laanwj/master

Change date format to ISO 8601