Add histograms comparing PBS unfolding with PBS + 0.5 M CaCl2

Add histograms comparing PBS unfolding with PBS + 0.5 M CaCl2

These are the histograms (and the unfolding force series from which
they are constructed) used in the sawsim fits on the following slide.
`diff --git a/Makefile b/Makefile
index 3e7e510..6fa0f2c 100644
--- a/Makefile
+++ b/Makefile
@@ -53,7 +53,8 @@ THUMBNAILS = \
 	$(DEST)/fit-valley-fw-fh.png \
 	$(DEST)/1TIT-hbond-fw-fh.png \
 	$(DEST)/fit-valley-PBS-hw-hh.png \
-	$(DEST)/fit-valley-PBS-0.5M-CaCl2-hw-hh.png
+	$(DEST)/fit-valley-PBS-0.5M-CaCl2-hw-hh.png \
+	$(DEST)/pbs-0.5M-CaCl2-hist-fw-fh.png

 #	$(DEST)/landscape-cant-hw-hh.png