Add a chemicalequationinput with live preview

- architecturally slightly questionable: the preview ajax calls goes to an LMS view instead of an input type specific one.  This needs to be fixed during the grand capa re-org, but there isn't time to do it right now.
- also, I kind of like having a generic turn-a-formula-into-a-preview service available

common/static/js/capa/README

@@ -0,0 +1 @@
+These files really should be in the capa module, but we don't have a way to load js from there at the moment.  (TODO)

common/static/js/capa/chemical_equation_preview.js

@@ -0,0 +1,12 @@
+(function () {
+    var preview_div = $('.chemicalequationinput .equation');
+    $('.chemicalequationinput input').bind("input", function(eventObject) {
+        $.get("/preview/chemcalc/", {"formula" : this.value}, function(response) {
+            if (response.error) {
+                preview_div.html("<span class='error'>" + response.error + "</span>");
+            } else {
+                preview_div.html(response.preview);
+            }
+          });
+    });
+}).call(this);