Use standard modifiedDate field instead of the custom property

 This adds a bit of extra debug printing that you can elide if necessary.

The main thing that is missing is support for parsing crazy dates outside of the 0000..9999 year range. (Drive supports this but time.Parse does not.)

I am a Google employee so no need for CLA.
It looks like this is fine in the end, as syncFileUp() ends up setting it, but it'd be nice to just have it correct starting at creation time.