Add support for PDF in figprint if gnuplot supports

Gnuplot support for pdf is enabled in homebrew with
`brew install gnu plot --pdf``
Document pdf for figprint
Send error if gnuplot does not have pdf support
Add function pdffigure()
Document pdf possibilities
Document in main description, too
Fix typo in description of eps terminal
Merge pull request #174 from jucor/pdfwrite

Add support for PDF if gnuplot supports