fix datepicker in modal. Fixes #505 #506

components/datepicker/demo/dialog.md

@@ -0,0 +1,45 @@
+# 结合弹出层
+
+- order: 10
+
+弹出层内的时间选择
+
+---
+
+````jsx
+import { Modal, Button, Datepicker } from 'antd';
+
+const App = React.createClass({
+  getInitialState() {
+    return { visible: false };
+  },
+  showModal() {
+    this.setState({
+      visible: true
+    });
+  },
+  handleOk() {
+    console.log('点击了确定');
+    this.setState({
+      visible: false
+    });
+  },
+  handleCancel() {
+    this.setState({
+      visible: false
+    });
+  },
+  render() {
+    return <div>
+      <Button type="primary" onClick={this.showModal}>显示对话框</Button>
+      <Modal title="第一个 Modal" visible={this.state.visible}
+      onOk={this.handleOk} onCancel={this.handleCancel}>
+        <Datepicker defaultValue="2015-12-12" popupStyle={{zIndex:2000}}/>
+      </Modal>
+    </div>;
+  }
+});
+
+ReactDOM.render(<App /> , document.getElementById('components-datepicker-demo-dialog'));
+````
+

style/components/datepicker/Picker.less

@@ -1,9 +1,6 @@
 .@{calendar-prefix-cls}-picker-container  {
   position: absolute;
-  .@{calendar-prefix-cls} {
-    position: absolute;
-    z-index: 1070;
-  }
+  z-index: 1070;
 }
 
 .@{calendar-prefix-cls}-picker {