add xcode project target

Use locally installed node-pre-gyp if there's no global one.
This is necessary to be able to run make outside of npm.

Make example executable.

Change path to the request include file.

Bump mbgl to master.

src/node_request.cpp

@@ -1,6 +1,6 @@
 #include "node_request.hpp"
 #include "node_file_source.hpp"
-#include <mbgl/storage/default/request.hpp>
+#include <mbgl/storage/request.hpp>
 #include <mbgl/storage/response.hpp>
 
 #include <cmath>