Fixes https://github.com/grpc/grpc/issues/2194 .

Part of fixing https://github.com/grpc/grpc/issues/2192

- Relax protobuf requirement to alpha version
- Suppress warnings for GRPCClient
- Fix headers search path
- Document hacks.

And solves a few linter issues that prevent it from being submitted to Cocoapods' repo.

So the root of the repository doesn’t have 2 podspecs.
All four libraries are subspecs now.
Adjusts the generated code to account for that.

- Moves them to the root of the repo.
- Makes them pass lint (except for the release tag warning).
- Includes the script to rename time.h and string.h in the spec.
- Fixes the #includes in the wrapper files.