Readd the commented-out config to suppress warnings, to have it handy
for submissions.

Tests not yet included.

Still missing:
- Caching of the already checked JWTs (although it could be done at an
  upper layer).
- Caching of the jwks_uri to avoid 2 roundtrips for each verification.

- uri parsing
- dns resolution

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.