Fix sorting by dateTaken and dateModified

dateTaken acquiring logic much improved (see last comment). We should fix sorting by dateTaken and dateModified.

Hello,

Grid’s is not able to acquire `dateTaken` correctly. Here is a bunch of examples converted using latest Lightroom: [PROD]search?query=~"dev blog"&uploadedBy=[me]@guardian.co.uk

All dates are (and, consequently, ordering) are wrongly set at:
`"dateTaken": "2016-06-10T00:00:00Z",`

We already read correct `dateTaken` which is recorded in multiple places inside `fileMetadata`, e.g.:

```
iptc": {
"Digital Time Created": "15:28:37",
"Digital Date Created": "20160610",
"Time Created": "15:28:37",
"Date Created": "Fri Jun 10 00:00:00 UTC 2016"
"exifSub": {
"Date/Time Original": "2016:06:10 15:28:37",
"Date/Time Digitized": "2016:06:10 15:28:37",
```

Recent [updates](https://github.com/drewnoakes/metadata-extractor/releases) to metadata-extractor cite

> Considerable overhaul of date, time and time zone handling across the library #153 #154 #155 #158

Maybe if we would to update it, we wouldn’t have to think too hard about how should we reconcile the dates ourselves?

FYI, full exiftool dump reveals:

```
---- ExifTool ----
ExifTool Version Number : 10.09
---- File ----

File Modification Date/Time : 2016:06:10 22:55:19+01:00
File Access Date/Time : 2016:06:10 22:17:27+01:00
File Creation Date/Time : 2016:06:10 22:17:27+01:00
---- EXIF ----
Modify Date : 2016:06:10 22:17:29
Date/Time Original : 2016:06:10 15:28:37
Create Date : 2016:06:10 15:28:37
---- XMP ----
Modify Date : 2016:06:10 22:17:29+01:00
Create Date : 2016:06:10 15:28:37
Metadata Date : 2016:06:10 22:55:19+01:00
Date Created : 2016:06:10 15:28:37
History When : 2016:06:10 22:17:29+01:00*2016:06:10 22:54:54+01:00*2016:06:10 22:55:19+01:00
---- IPTC ----
Date Created : 2016:06:10
Time Created : 15:28:37
Digital Creation Date : 2016:06:10
Digital Creation Time : 15:28:37
---- Composite ----
Date/Time Created : 2016:06:10 15:28:37
Digital Creation Date/Time : 2016:06:10 15:28:37
```

Regards
Mateusz