Extracted the WebUpdate example into a library.

This ESP8266HTTPUpdateServer can be instantiated and used
more cleanly, it's also able to take or create an ESP8266WebServer
to configure the events and /update handler.

It's been made more robust by handling upload abort, which
depends on fix provided for issue #833

libraries/ESP8266HTTPUpdateServer/src/ESP8266HTTPUpdateServer.h

@@ -0,0 +1,18 @@
+#ifndef __HTTP_UPDATE_SERVER_H
+#define __HTTP_UPDATE_SERVER_H
+
+class ESP8266WebServer;
+
+class ESP8266HTTPUpdateServer
+{
+  private:
+    bool _serial_output;
+    ESP8266WebServer *_server;
+    static const char *_serverIndex;
+  public:
+    ESP8266HTTPUpdateServer(bool serial_debug=false);
+    void setup(ESP8266WebServer *server=NULL);
+};
+
+
+#endif