added bug fix for hide-before-show bug

this commit fixes a bug where .modal('hide') is called on a modal object before
.modal('show').  this can be viewed here:

    http://jsfiddle.net/aaronj1335/7hH89/8/

(notice the JS exception that is thrown)

js/bootstrap-modal.js

@@ -100,6 +100,8 @@
     , hide: function (e) {
         e && e.preventDefault()
 
+        if (! this.isShown) return
+
         var that = this
         this.isShown = false