- The Timestamp proto does not allow for negative nanos fields, so the seconds
  must be shifted and a positive nanos then applied.
- Tweak the helpers on Duration to make it clear there is no "base" time
  involved.
- Update the unittests for duration and timestamp to cover positive and
  negative NSTimeIntervals and what their impact is on the protos.

The Timestamp proto does not allow for negative nanos fields, so the seconds must be shifted and
a positive nanos then applied.

- Capture the ObjC prefix used when generating the the file.
- Track the containing type on descriptors.
- Mark descriptors where the message class name got a suffix added to it.
- Expose a fullName property on Descriptors.
- Add helpers for packing/unpacking Any messages.
- Bump the ObjC runtime version number. Since we added methods and invoke them
  in the generated code, ensure the code is running against a matching version.
  Otherwise, someone could compile against headers, but run with a framework
  that is older and get unknown selector failures.  This should trip clearer
  messaging.

Fixes https://github.com/google/protobuf/issues/1674

As bazel folks are looking at getting auto generation of module maps going and
the importing of sources files causes issues there.  We were only do it to
hack around some of the apple linker behaviors around objc classes and
categories, but even that isn't complete and CocoaPods was already doing -ObjC,
and developers not using pods could have still needed it to ensure everything
was linked anyways; so drop the hack of importing sources.

This reverts commit 7366efd8.

Still some discussion about the api to expose the helpers with.

fix an bug(forgive my english..)

The previous two methods make it easy to transform between any and normal message.
unPackeTo will throw error if the type url in any doesn't match the type of the message to be transformed to.
is checks any's type url matches the give GPBMessage type.