PG language enhancements

• PGalias.pm -- finding external resources for a question
  • (PGload.pm does something similar searching for macro files)
  • findAppletCodebase() also does something similar
  • use unique ID file names -- so you can't snoop things like psvn numbers, login names and so forth
  • factor code so that a greater variety of resource types can be added easily currently we have .html, .png, .gif -- I think svg, jpeg, could be added and ???
  • save the paths to resources used so that we can add some facility to download the resources as well as the question on export -- this will facilitate sharing problems.
  • if applets are referenced they should be indexed in a similar manner.

• Answer blank handling -- PGanswergroup.pm, PGresponsegroup.pm

• Update and consolidate the handling of matrices
  • Fix the coloring of answer blanks so that works with matrices as well.