Firestack compatibility (#81)

* Add firestack header search paths

If users have installed firestack, modules may live here instead.
Include them just in case.

* Include firestack paths to frameworks

* Import Firebase via module

* Ignore xcuserdata

.gitignore

 node_modules
 build
+xcuserdata

ios/RNFIRMessaging.h

 
 #import <UIKit/UIKit.h>
 
-#import "Firebase.h"
+@import Firebase;
+@import FirebaseMessaging;
 
 #import "RCTBridgeModule.h"
 

ios/RNFIRMessaging.xcodeproj/project.pbxproj

@@ -212,11 +212,13 @@
 				FRAMEWORK_SEARCH_PATHS = (
 					"$(inherited)",
 					"$(PROJECT_DIR)/../../../ios/**",
+					"$(SRCROOT)/../../../node_modules/react-native-firestack/ios/**",
 				);
 				GCC_PREPROCESSOR_DEFINITIONS = "$(inherited)";
 				HEADER_SEARCH_PATHS = (
 					"$(SRCROOT)/../../react-native/React/**",
 					"$(SRCROOT)/../../../ios/**",
+					"$(SRCROOT)/../../../node_modules/react-native-firestack/ios/**",
 				);
 				LIBRARY_SEARCH_PATHS = "$(inherited)";
 				OTHER_LDFLAGS = "-ObjC";
@@ -233,10 +235,12 @@
 				FRAMEWORK_SEARCH_PATHS = (
 					"$(inherited)",
 					"$(PROJECT_DIR)/../../../ios/**",
+					"$(SRCROOT)/../../../node_modules/react-native-firestack/ios/**",
 				);
 				HEADER_SEARCH_PATHS = (
 					"$(SRCROOT)/../../react-native/React/**",
 					"$(SRCROOT)/../../../ios/**",
+					"$(SRCROOT)/../../../node_modules/react-native-firestack/ios/**",
 				);
 				LIBRARY_SEARCH_PATHS = "$(inherited)";
 				OTHER_LDFLAGS = "-ObjC";