This change makes use of new imports attribute for Bazel's Python rules, which
enable adding directories to the PYTHONPATH. This allows us to remove
the hack for building protobuf's Python support with Bazel and now
allows projects to include protobuf using a Bazel external repository
rather than requiring it to be imported directly into the source tree as
//google/protobuf.

This change also updates the protobuf BUILD file to use a named
repository, @python//, for including Python headers rather than
//util/python. This allows projects to specify their own package for
Python headers when including protobuf with an external repository.

Fixes #1230

grpc C++ plugin generates additional files, namely .grpc.pb.cc and
.grpc.pb.h. Adding these files to the outs of the _proto_gen rule, so
dependents don't complain about undeclared inclusions. Also, compiling
the .grpc.pb.cc requires additional header files from the grpc library,
so added //external:grpc_lib to the deps of the
cc_library. Clients are expected to declare that in their bazel
WORKSPACE, pointing it to @grpc//:grpc++{_unsecure}.

cc_proto_library now supports use_grpc_plugin flag that passes
--plugin=protoc-gen-grpc=grpc_cpp_plugin to protoc compiler
invocation. grpc_cpp_plugin is assumed to be present as
//external:grpc_cpp_plugin, so clients can setup their WORKSPACE files
appropriately to point to grpc location using bind.

This change make protobuf skylark extension works when using
remote repository.

Note that this make the Skylark extension unusable prior
to Bazel 0.1.4 because the workspace_root is not available
on prior version.

Tested with Bazel 0.1.4rc2.

Fixes https://github.com/bazelbuild/bazel/issues/784.

This enables other xx_proto_library targets to depend on xx_proto_library
targets in different packages, and specifically on xx_wkt_protos.

This is necessary to run protoc on the host as a dependency for Android BUILD targets with Bazel.

To mention that the interface may change or be removed when bazel has
support it natively.

This adds implicit dependencies to the coressponding runtime libraries.

So that the rule fails if one or more files cannot be copied.

We need to use the list to indicate field presense.
The field must only contain 0 or 1 string element.

This enables the use case where all the paths are relative to the
workspace root, e.g.

foo/bar/BUILD
       /foo.proto -- package foo.bar

would generate the message correctly.

This is needed to support python bazel build.