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
Allow arbitrary filename in summary buffer
Merge pull request #61 from yoshinari-nomura/allow_arbitrary_filename

Allow arbitrary filename in summary buffer
Add variables for pamscale
Use variables instead of fixed string
Merge pull request #62 from yuutayamada/pamscale

Add variables for pamscale/pnmscale
temporary fix for pnm.
Using "pnmscale" by default.