fixed db-blob.cpp to not crash if no changes applied

The db-blob tool crashes if no changes are applied. This fixes the issue.
fixed a bug in db/file/Track

There is a bug in Track.hpp when reading blobs like "1a2b3c -- some.mp3". After
reading the blob ref, the trailing whitespace is skipped, which leaves
"-- some.mp3" in the buffer. However, the format reading part reads until " -- "
separator. Fixed by ignoring the last character in ctmp. Also improved
exceptions a bit to be more informative.
db-add: metadata fields lowercase

When db-add.cpp creates a track entry, the metadata field names are now in
lowercase.
db-blob: added -a option

The db-blob.cpp tool may now be used to add and delete blobs. Also improved the
tool's output a bit, making it more machine-friendly.