Adds aria described by to tooltip plugin for accessibility

Generates a unique id for tooltip and adds [aria-describedby] to the element
it is called on. Resolves issue #13480

- set up test
- linted the code styles
- passed the tests
- integrated feedback from @cvrebert

dist/js/bootstrap.js

@@ -1233,7 +1233,11 @@ if (typeof jQuery === 'undefined') { throw new Error('Bootstrap\'s JavaScript re
 
       var $tip = this.tip()
 
+      var tipId = this.getUID(this.type)
+
       this.setContent()
+      $tip.attr('id', tipId)
+      this.$element.attr('aria-describedby', tipId)
 
       if (this.options.animation) $tip.addClass('fade')
 
@@ -1356,6 +1360,8 @@ if (typeof jQuery === 'undefined') { throw new Error('Bootstrap\'s JavaScript re
     var $tip = this.tip()
     var e    = $.Event('hide.bs.' + this.type)
 
+    this.$element.removeAttr('aria-describedby')
+
     function complete() {
       if (that.hoverState != 'in') $tip.detach()
       that.$element.trigger('hidden.bs.' + that.type)
@@ -1447,6 +1453,12 @@ if (typeof jQuery === 'undefined') { throw new Error('Bootstrap\'s JavaScript re
     return title
   }
 
+  Tooltip.prototype.getUID = function (prefix) {
+    do prefix += ~~(Math.random() * 1000000)
+    while (document.getElementById(prefix))
+    return prefix
+  }
+
   Tooltip.prototype.tip = function () {
     return this.$tip = this.$tip || $(this.options.template)
   }