Assume http:// as default for PDF import from web

If URL is not parsable, try adding “http://“ and see if that  works,
rather than immediately throwing an error (“Error! I, Librarian could
not connect with an external web service.”…)
Mac install instruction

Clarify and extend instructions for installing on MacOS.
Provide simultaneous links to both PDF viewers

If pdfviewer is set to internal, include links to both the internal and
external viewers in the file_top.