Add "failed" message when pdftotext does not exist

This is about displaying a PDF attached file.
Merge pull request #59 from trueroad/no-pdftotext-message

Add "failed" message when pdftotext does not exist
Support stunnel 5.x
Merge pull request #60 from tats/bug/stunnel5

Support stunnel 5.x
Merge branch 'master' into nagao-oresama-branch