isETagGood IIS7+ compatible

https://github.com/marcelduran/yslow/issues/61

http://technet.microsoft.com/en-us/library/ee619764(v=WS.10).aspx
"In IIS 7.0, the metabase change number of 0 is the default setting and cannot be modified."

This will make the default IIS 7.0+ ETAG CDNs compatible as Timestamp:0 can be used across multiple IIS CDNs.

    - var reIIS = /^[0-9a-f]+:[0-9a-f]+$/,
    + var reIIS = /^[0-9a-f]+:([1-9a-f]|[0-9a-f]{2,})$/,