Changing the credentials plugin API.

Also passing empty string as opposed to NULL when the method name is not
found. This is much less error-prone.

The purpose of this is to be able to install a composition policy that
describes which types are incompatible and that will be enforced during
call creds composition.

If this functionality is wanted it will be done in an additive function in
the API like :

void grpc_call_credentials_set_composite_policy(
  grpc_call_credentials_composite_policy policy);

Updated Node package version to 0.12.0

bump C# version to 0.12.0

- The plugin is now passed more information that it can use to create
  auth metadata:
    - service_url (as before)
    - method name
    - channel_auth_context

Core credentials API refactoring

Fixing NULL terminator for test cert.

Forgot to do that when I replaced the cert in #4096. Thanks asan for
catching that!

Updating the server1 cert so that it can be used with Go.

Merge release-0_11 into master

Handle Cancel before start.

Add missing grace value to server stop in Python example.

Update Python C wrapping layers to new credentials API

Fix dead link in doc

Generate all file lists and build target lists in binding.gyp from build.yaml

Fix naming issue in stress_test

(via ahh@google.com)
Under some configurations gRPC includes google3 base headers.  google3
has a namespace "thread" which mixes poorly with unqualified use of the
type std::thread; adding that namespace to various google3 headers
breaks this test.

Improve grammar/language for clarity

Fixes #4122

Add a useful __str__ method to face exceptions

Small typo of NodeJS version in README.md