Add examples 15, 17, 18, and 19.
Add examples 15, 17, 18, and 19.

Note 17 and 19 don't link properly yet (missing fortran API) so the
installed examples will have to be built using make -k.

svn path=/trunk/; revision=4989

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot