title: d3-tip doesn't work with webpack
body: <p>I use webpack and TypeScript and it seems that d3-tip isn't able to work with webpack. I get error on mouseover events 
  <code>"Uncaught TypeError: Cannot read property 'target' of null"</code>.</p>
  
  <p>This error occurs because d3.event in the d3-tip module is null.</p>
  
  <p>I include modules as follows:</p>
  
  <pre><code>const d3: any = require("d3");
  d3.tip = require("d3-tip");
  </code></pre>
  
  <p>but I guess that d3 there and d3 in the d3-tip module are different and this is the source of problems, but I don't know how to solve it. In the d3-tip module we have:</p>
  
  <pre><code>(function (root, factory) {
      if (typeof define === 'function' &amp;&amp; define.amd) {
          // AMD. Register as an anonymous module with d3 as a dependency.
          define(['d3'], factory)
      } else if (typeof module === 'object' &amp;&amp; module.exports) {
          // CommonJS
          var d3 = require('d3')
          module.exports = factory(d3)
      } else {
          // Browser global.
          root.d3.tip = factory(root.d3)
      }
  }(this, function (d3) {
  ...
  </code></pre>
  
  <p>And it compiles by webpack into</p>
  
  <pre><code>function(module, exports, __webpack_require__) {
  
  var __WEBPACK_AMD_DEFINE_FACTORY__, __WEBPACK_AMD_DEFINE_ARRAY__, __WEBPACK_AMD_DEFINE_RESULT__;// d3.tip
  // Copyright (c) 2013 Justin Palmer
  //
  // Tooltips for d3.js SVG visualizations
  
  (function (root, factory) {
      if (true) {
          // AMD. Register as an anonymous module with d3 as a dependency.
          !(__WEBPACK_AMD_DEFINE_ARRAY__ = [__webpack_require__(465)], __WEBPACK_AMD_DEFINE_FACTORY__ = (factory), __WEBPACK_AMD_DEFINE_RESULT__ = (typeof __WEBPACK_AMD_DEFINE_FACTORY__ === 'function' ? (__WEBPACK_AMD_DEFINE_FACTORY__.apply(exports, __WEBPACK_AMD_DEFINE_ARRAY__)) : __WEBPACK_AMD_DEFINE_FACTORY__), __WEBPACK_AMD_DEFINE_RESULT__ !== undefined &amp;&amp; (module.exports = __WEBPACK_AMD_DEFINE_RESULT__))
      } else if (typeof module === 'object' &amp;&amp; module.exports) {
          // CommonJS
          var d3 = require('d3')
          module.exports = factory(d3)
      } else {
          // Browser global.
          root.d3.tip = factory(root.d3)
      }
  }(this, function (d3) {
  ...
  </code></pre>
  
  <p>and it's obvious that AMD is using. If I could get the factory of d3-tip I would solve that problem.</p>