fix c++ readme and tutorial

This should have been included in 46585e23.

The SwiftSample doesn’t build in this commit, but I need to sync with
head anyway so I’ll first merge v1.0.x into this branch and then debug.

Update node protobuf dependency to 3.0.0 where applicable. Also update example dependency to grpc 1.0.0

To use the 3.0.0-beta-4 version of protoc, which has the fix for oneofs
when using dynamic frameworks.

Which has the fix for oneofs when using dynamic frameworks in ObjectiveC

It’s the magic sentence that allows pods dependent on RxLibrary to be
archived correctly by XCode.

It’s less than ideal, and seems arbitrary (why RxLibrary specifically?),
so we’ll try to produce a minimal case and open an issue with it in the
Cocoapods repo.

Pending:

- Tag BoringSSL repo.
- Tag gRPC repo.
- Build the plugin, zip it, and add it to the release.
- Push Cocoapods to trunk.

I wasted a huge amount of time debugging this madness.

The examples under <repo_root>/examples now rely on the released
versions. Those under src/objective-c/examples, as well as the tests,
rely on protoc and the plugin as compiled from head.

It’s just the merge of beta-3.1 and beta-3.2, both of which we need.