Generate imports for .proto dependencies, and read prefix from file.

The prefix has still to be applied per-message, and we could
do forward-declarations in the generated header.
diff --git a/src/compiler/objective_c_generator.cc b/src/compiler/objective_c_generator.cc
index 6898b4d..8f35302 100644
--- a/src/compiler/objective_c_generator.cc
+++ b/src/compiler/objective_c_generator.cc
@@ -118,9 +118,9 @@
                                const MethodDescriptor *method,
                                map<string, string> vars) {
   printer->Print("{\n");
-  printer->Print(vars, "[[self RPCTo$method_name$With");
+  printer->Print(vars, "  [[self RPCTo$method_name$With");
   if (method->client_streaming()) {
-    printer->Print("RequestsWriter:requestsWriter"); //TODO(jcanizales):request?
+    printer->Print("RequestsWriter:request");
   } else {
     printer->Print("Request:request");
   }
@@ -136,12 +136,13 @@
 
   printer->Print("            requestsWriter:");
   if (method->client_streaming()) {
-    printer->Print("requestsWriter\n");
+    printer->Print("request\n");
   } else {
     printer->Print("[GRXWriter writerWithValue:request]\n");
   }
 
-  printer->Print(vars, "             responseClass:[$response_type$ class]\n");
+  printer->Print(vars,
+      "             responseClass:[$prefix$$response_type$ class]\n");
 
   printer->Print("        responsesWriteable:[GRXWriteable ");
   if (method->server_streaming()) {