Define type for title property in PageItem

Accorging to http://www.w3.org/TR/WCAG20-TECHS/H33.html
The title attribute is used to provide additional information to help clarify
or further describe the purpose of a link.

Еherefore I set type for `title` to `string`.

Author: AlexKVal

src/PageItem.js

@@ -6,6 +6,7 @@ const PageItem = React.createClass({
   propTypes: {
     href: React.PropTypes.string,
     target: React.PropTypes.string,
+    title: React.PropTypes.string,
     disabled: React.PropTypes.bool,
     previous: React.PropTypes.bool,
     next: React.PropTypes.bool,