master.tex: Insert first page with instructions for students and a last
	    page where students mark their answers to take with them.
master.tex: renamed sample anser key jpeg inserted in the test's first
	    page to front-gabarito.jpg
front-gabarito.jpg: sample answer key jpeg to be in the first page with
		    instructions to the test.
grade.pl: Fixed name of input file (from optical reader). Now it has the
	  form mat-2457-p$prova-t$class.dat.
sample1.dat, sample2.dat: Rescetively renamed to mat2457-p1-t1.dat and
	     		  mat2457-p2-t1.dat.
mat2457-p1-t1.dat, mat2457-p2-t1.dat: These are the new names for
		   		      sample1.dat and sample2.dat,
		   		      respectively.
grade.pl: Removed a chekpoint line in function hdr_print() function.