Simpler client creation, secure connections are now the default.

Previous versions required a separate "roots.pem" file to be loaded to initialize secure connections. Now this file is built into the library binary (see Sources/gRPC/Roots.swift) so clients no longer have to worry about providing this file.

In a related simplification, the Channel constructor now takes an optional "secure" argument whose default value is true. Note that this will break code that calls the Channel constructor with just an address since previously that created insecure connections. To get an insecure channel, add "secure:false" to your Channel constructor.