Avoid trailing whitespace in the JSON output

The default separators used in the Python JSON library's dump() write
output with trailing whitespace. Use custom separators to avoid that, as
described in the documentation:

  http://docs.python.org/2/library/json.html
Make image filenames relative to the label file

The image filenames written to the label files were absolute pathnames.
Change this to use filenames relative to the label file.
Append only new files

Append only new files in AppendFilesCommand(), i.e. check if the file
is already in the label file before appending.
Merge branch 'master' of github.com:neingeist/sloth