Fix error if debug is set to false

If debug was switch to false ditaa will be run in the background
and causes an error since the temp file was already deleted.

Signed-off-by: Marc Koderer <m.koderer@telekom.de>
fix tempfile handling, use class Tempfile

Use Tempfile for automated random file names under /tmp and cleanup.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #6 from dalgaaf/fix_png_generation

Fix ditaa PNG generation