Improve time zone and Geo coordinate parsing

This is a merge with
camlistore.org/third_party/github.com/rwcarlsen/goexif

Squash of the following commits:

- exif: make DateTime return time.Local location when unknown timezone
  (https://camlistore.googlesource.com/camlistore/+/a82738064436e5010268616655650ac71855668f)
- exif: add Exif.LatLong accessor
  https://camlistore.googlesource.com/camlistore/+/b6d3d1a1037260b753b914e0c07279811cc76777
- Prevent panic crash due to malformed EXIF geotags
  (https://camlistore-review.googlesource.com/#/c/3637/5)