make knit_to_pdf work with rnw files not in working directory
make knit_to_pdf work with rnw files not in working directory

fboehm/labnotebook,fboehm/labnotebook,fboehm/cboettig_site,antass/labnotebook3,fboehm/cboettig_site,fboehm/labnotebook,antass/labnotebook3,fboehm/labnotebook,fboehm/cboettig_site,fboehm/cboettig_site,antass/labnotebook3,fboehm/cboettig_site,fboehm/labnotebook,fboehm/labnotebook