WIP commit for iOS:

- failure handler gets called if an exception occurs in native code
- rename "raw" serializer to "utf8"
- extract serializer into separate file

src/ios/TextRequestSerializer.h

@@ -0,0 +1,8 @@
+#import <Foundation/Foundation.h>
+#import "AFURLRequestSerialization.h"
+
+@interface TextRequestSerializer : AFHTTPRequestSerializer
+
++ (instancetype)serializer;
+
+@end