Improve react-native-xcode.sh integration

Summary:
Inspired by conversation in facebook/react-native#5374, this PR improves `react-native-xcode.sh`:

* No longer depends on global `react-native` binary
* Gracefully handles missing `node` dependency and adds a new way to configure the path to `node` in non-standard installation environments

This is how the error looks like:
![image](https://cloud.githubusercontent.com/assets/192222/12538882/3f9b5c3e-c29a-11e5-84fc-c7ccedf1c46a.png)
Closes facebook/react-native#5518

Reviewed By: svcscm

Differential Revision: D2861116

Pulled By: frantic

fb-gh-sync-id: 9a80eda6c844d066e34369b1cda503955171485b

Author: frantic

generator-ios/templates/xcodeproj/project.pbxproj

@@ -525,7 +525,7 @@
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 			shellPath = /bin/sh;
-			shellScript = "../node_modules/react-native/packager/react-native-xcode.sh";
+			shellScript = "export NODE_BINARY=node\n../node_modules/react-native/packager/react-native-xcode.sh";
 			showEnvVarsInLog = 1;
 		};
 /* End PBXShellScriptBuildPhase section */