Support for listviews with sections and further new features...

Adding support for

1) listviews with sections by setting the property "withSections" to "true" and specifying a method for renderSectionHeader
2) Triggering a refresh from external by adding "onRefresh()" which calls "_onRefresh()" with an additional parameter
3) Calling "_onFetch()" with a third parameter called "options" which is an object which is currently either {external: true} or {initial: true} or {}, depending on whether the call of the _onFetch() method was triggered from external (i.e. by calling onRefresh()), was the initial creation of the listview (i.e. from componentDidMount) or any other call.

Author: PhilippKrone

GiftedListView.js

@@ -70,7 +70,7 @@ var GiftedListView = React.createClass({
       refreshable: true,
       refreshableViewHeight: 50,
       refreshableDistance: 40,
-      onFetch(page, callback) { callback([]); },
+      onFetch(page, callback, options) { callback([]); },
 
       paginationFetchigView() {
         return (
@@ -241,7 +241,7 @@ var GiftedListView = React.createClass({
     this.setState({
       paginationStatus: 'fetching',
     });
-    this.props.onFetch(this._getPage() + 1, this._postPaginate);
+    this.props.onFetch(this._getPage() + 1, this._postPaginate, {});
   },
 
   _postPaginate(rows = [], options = {}) {