Merge upstream goexif

This pulls the changes from the current HEAD of
https://github.com/rwcarlsen/goexif
(eb2943811adc24a1a40d6dc0525995d4f8563d08)

Notable changes:
- Removed explicit panics in favor of error returns
- renamed TypeCategory to Format and made format calculated upon
  decoding rather than repeatedly for every format call
- Merged contributions from Camlistore (exif.LatLong(), exif.DateTime()
  etc.)
- Change String method to just return the string value - and don't have
  square brackets if only a single value
- add separate Int and Int64 retrieval methods
- Doc updates

Minor changes in camlistore.org/pkg/* were neccessary to reflect
changes in the API (handling of returned errors) and in names of
exported fields and methods.

Change-Id: I50412b5e68d2c9ca766ff2ad1a4ac26926baccab
Merge "Merge upstream goexif"
Add sorted/leveldb

Based on github.com/syndtr/goleveldb/leveldb.

Change-Id: I4baf36fa0f6204c66816c7548952af72652f2589