Wed Sep 6 2017

This is a significant (possibly overreaching) update that brings grpc
support up to latest. rules_protobuf should also load much faster
than previously. Some of the names of external workspaces have
changed that may break your build (for example, string references such
as 'external/com_github_google_protobuf/src' should be migrated to
'external/com_google_protobuf/src' in an 'imports' attribute).

• Introduce cpp/grpc_repository.bzl to setup a custom @com_google_grpc
  external workspace (no longer using git_repository#init_submodules).
  This pulls down grpc/grpc from github as @com_google_grpc_base, sets
  up a mirrored external workspace in @com_google_grpc that symlinks
  to @com_google_grpc_base, then does a "git submodule the hard way" for
  c-ares, and installs a patched version of generate_cc.bzl.
• Rename repository @com_github_google_protobuf to @com_google_protobuf.
• Rename repository @com_github_grpc_grpc to @com_google_grpc.
• Rename repository @gtest to @com_google_googletest.
• Updated grpc/grpc to 1.6.1.
• Updated madler/zlib to 1.2.11.
• Updated boringssl to master-with-bazel Sep 2 2017.
• Updated nuget deps to 1.6.0.
• Adding binding to //external:protocol_compiler (that's what with grpc repo wants).
• Remove binding //external:protoc (now //external:protocol_compiler).
• Migrated all git_repository repository_rules to http_archive
  (faster).