Add support for PDF if gnuplot supports

This pull-request provides both gnuplot.pdffigure() and
gnuplot.figprint('whatever.pdf'). 

If gnuplot has not been compiled with pdf support, an error
is raised when either of these functionalities are called.
Gnuplot support for pdf is enabled on OSX with homebrew using
`brew install gnu plot --pdf`

Besides, if the terminal `pdfcairo` is available, it is used instead
of the `pdf` terminal