Add option to append actionButton to custom elt

Added optional parameter to createActionButton to specify a page element
to which it should be appended. This is needed for the manual tests so
that we can group action buttons logically with info on the page. Defaults
to 'buttons' div as before.
diff --git a/www/main.js b/www/main.js
index a39a6bc..6899166 100644
--- a/www/main.js
+++ b/www/main.js
@@ -129,8 +129,9 @@
 
 /******************************************************************************/
 
-function createActionButton(title, callback) {
-  var buttons = document.getElementById('buttons');
+function createActionButton(title, callback, appendTo) {
+  appendTo = appendTo ? appendTo : 'buttons';
+  var buttons = document.getElementById(appendTo);
   var div = document.createElement('div');
   var button = document.createElement('a');
   button.textContent = title;