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.
There's an interesting philosophical question in this test that basically boils down to, "is this an upload/download utility, or is it a sync client?".  I had originally written this as !After since in the case where machine A uploaded a file to drive, machine B uploaded a newer version of the file, and then machine A uploaded again, A's version probably shouldn't clobber the version on Drive.

However, skicka's readme is all about "this isn't a sync client (yet); it's an upload/download utility".  So following the rule of "when the upload/download is done, the local files and remote files match" is probably the right thing.

tl;dr, I think this is an ok change, but would be happy to hear confirmation that your reasoning fits with the above.