Add getItemValue to propTypes (#103)

Author: Will Clark

lib/Autocomplete.js

@@ -10,6 +10,7 @@ let Autocomplete = React.createClass({
     onChange: React.PropTypes.func,
     onSelect: React.PropTypes.func,
     shouldItemRender: React.PropTypes.func,
+    getItemValue: React.PropTypes.func.isRequired,
     renderItem: React.PropTypes.func.isRequired,
     menuStyle: React.PropTypes.object,
     inputProps: React.PropTypes.object,