Add Bazel target for protobuf ObjC runtime

Also add WKT headers to the umbrella file, and simplify Podspec with it.
Plus some layout improvements to the BUILD file.

objectivec/GPBProtocolBuffers.h

diff --git a/objectivec/GPBProtocolBuffers.h b/objectivec/GPBProtocolBuffers.h
index a66f295..0c85259 100644
--- a/objectivec/GPBProtocolBuffers.h
+++ b/objectivec/GPBProtocolBuffers.h
@@ -42,3 +42,15 @@
 #import "GPBUnknownFieldSet.h"
 #import "GPBUtilities.h"
 #import "GPBWireFormat.h"
+
+// Well-known proto types
+#import "google/protobuf/Any.pbobjc.h"
+#import "google/protobuf/Api.pbobjc.h"
+#import "google/protobuf/Duration.pbobjc.h"
+#import "google/protobuf/Empty.pbobjc.h"
+#import "google/protobuf/FieldMask.pbobjc.h"
+#import "google/protobuf/SourceContext.pbobjc.h"
+#import "google/protobuf/Struct.pbobjc.h"
+#import "google/protobuf/Timestamp.pbobjc.h"
+#import "google/protobuf/Type.pbobjc.h"
+#import "google/protobuf/Wrappers.pbobjc.h"