[img.pixels] fix misc bugs in img.pixels loader

Following bugs are fixed:
 - Exceptions in scikit image reading the image are caught, and logged,
   and then opencv's imgread is used. This means we'll get broken JPEGs
   as a matrix. This may be a Bad Idea.
 - OpenCV's imread returns BGR, not RGB. The .pixels should always be
   RGB, so we invert this on the fly
[data/zeros] add zeros.jpg and truncated.jpg

Add files to test the reading of numpy arrays. Also add a truncated
jpeg to try crashing various readers.
[ts.util.imgmeta] Add get_exif_date function
[TSImage] get datetime from exif as fallback

If we add an image not from a timestream, the datetime parser failed. So
now we fall back to using get_exif_date to find the date from the exif
header.
[ts.util.imgmeta.get_exif_date] rewrite function

Now, we use get_exif_tag, and ensure we return None if we can't get the
date