Replace exiftool with taglib#

Main reason is that the previous fix for non-ascii characters in
filenames, involving pumping the data into standardInput and reading
from standardOutput, was crashing for many (typically larger) images.

The new code is cleaner, shorter, doesn't require starting a new
process, backwards compatible, and more likely to be cross-platform.

As a bonus two previously disabled unit tests now pass, and it seems
to be picking up more pre-existing metadata.

One downside is that current releases of programs built using the old
exiftool version will not be able to read all the metadata written by
the new version. Details are in the Metadata class comment.