Pushing update reason for gh-pages and bower.

e.g. mozilla/pdf.js@eabbfd78d1fd2301c1bf5ac10bd76a5829150574

make.js

@@ -265,11 +265,13 @@ target.web = function() {
         GH_PAGES_DIR + '/getting_started/index.html');
     echo('Done building with wintersmith.');
 
+    var reason = process.env['PDFJS_UPDATE_REASON'];
     cd(GH_PAGES_DIR);
     exec('git init');
     exec('git remote add origin ' + REPO);
     exec('git add -A');
-    exec('git commit -am "gh-pages site created via make.js script"');
+    exec('git commit -am "gh-pages site created via make.js script" -m ' +
+         '"PDF.js version ' + VERSION + (reason ? ' - ' + reason : '') + '"');
     exec('git branch -m gh-pages');
 
     echo();
@@ -354,7 +356,8 @@ target.dist = function() {
   echo('### Commiting changes');
 
   cd(DIST_DIR);
-  var message = 'PDF.js version ' + VERSION;
+  var reason = process.env['PDFJS_UPDATE_REASON'];
+  var message = 'PDF.js version ' + VERSION + (reason ? ' - ' + reason : '');
   exec('git add *');
   exec('git commit -am \"' + message + '\"');
   exec('git tag -a v' + VERSION + ' -m \"' + message + '\"');