Add support for exporting Swift modules

Summary:
External modules are any Objective-C class in which the implementation is private. This currently will be most useful for Swift classes but also has potential to allow exposing methods on 3rd party libraries to the bridge.
Closes #982
Github Author: Robert Payne <robertpayne@me.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Base/RCTBridgeModule.h

@@ -73,12 +73,67 @@ typedef void (^RCTResponseSenderBlock)(NSArray *response);
  * { ... }
  */
 #define RCT_REMAP_METHOD(js_name, method) \
+  RCT_EXTERN_REMAP_METHOD(js_name, method) \
+  - (void)method
+
+/**
+ * Use this macro in a private Objective-C implementation file to automatically
+ * register an external module with the bridge when it loads. This allows you to
+ * register Swift or private Objective-C classes with the bridge.
+ *
+ * For example if one wanted to export a Swift class to the bridge:
+ *
+ * MyModule.swift:
+ *
+ *   @objc(MyModule) class MyModule: NSObject {
+ *
+ *     @objc func doSomething(string: String! withFoo a: Int, bar b: Int) { ... }
+ *
+ *   }
+ *
+ * MyModuleExport.m:
+ *
+ *   #import "RCTBridgeModule.h"
+ *
+ *   @interface RCT_EXTERN_MODULE(MyModule, NSObject)
+ *
+ *   RCT_EXTERN_METHOD(doSomething:(NSString *)string withFoo:(NSInteger)a bar:(NSInteger)b)
+ *
+ *   @end
+ *
+ * This will now expose MyModule and the method to JavaScript via
+ * `NativeModules.MyModule.doSomething`
+ */
+#define RCT_EXTERN_MODULE(objc_name, objc_supername) \
+  RCT_EXTERN_REMAP_MODULE(, objc_name, objc_supername)
+
+/**
+ * Similar to RCT_EXTERN_MODULE but allows setting a custom JavaScript name
+ */
+#define RCT_EXTERN_REMAP_MODULE(js_name, objc_name, objc_supername) \
+  objc_name : objc_supername \
+  @end \
+  @interface objc_name (RCTExternModule) <RCTBridgeModule> \
+  @end \
+  @implementation objc_name (RCTExternModule) \
+  RCT_EXPORT_MODULE(js_name)
+
+/**
+ * Use this macro in accordance with RCT_EXTERN_MODULE to export methods
+ * of an external module.
+ */
+#define RCT_EXTERN_METHOD(method) \
+  RCT_EXTERN_REMAP_METHOD(, method)
+
+/**
+ * Similar to RCT_EXTERN_REMAP_METHOD but allows setting a custom JavaScript name
+ */
+#define RCT_EXTERN_REMAP_METHOD(js_name, method) \
   - (void)__rct_export__##method { \
     __attribute__((used, section("__DATA,RCTExport"))) \
     __attribute__((__aligned__(1))) \
     static const char *__rct_export_entry__[] = { __func__, #method, #js_name }; \
-  } \
-  - (void)method
+  }
 
 /**
  * Deprecated, do not use.