Remove hack for building Python support with Bazel.

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

Fixed grpc C++ plugin support.

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}.

Add a modified patch from craigcitro@ to handle namespace sharing.

Added grpc plugin support to cc_proto_library.

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.

Require VS2015 in the solution file

Add atomics support for 32-bit PPC.

Added support for CommonJS require()

Support ObjC Generic Collections

Validate that end-group tags match their corresponding start-group tags

Some fixes for the most recent release notes.

- Extend GPB*ObjectDictionary to support generic syntax.
- Update the generator to output generics so the enclosed type is exposed for compiler checks.
- Use generics in a the public interfaces.
- Update the generated sources that are checked in.

"nodejs" does not exist on Travis, it appears.

List python_cpp as failing on OS X

Disable the xctool updates

In light of https://github.com/google/protobuf/issues/1232, disable the xctool
update so we stay on 0.2.7 which seems to work better.

Remove the passing of -newSimulatorInstance since the older xctool doesn't
support it.

Generate C# code whenever descriptor.proto changes

Added ALLOW_UNLIMITED_BINARY_PROTO macro and setting it when

Add more tests around merging wrappers

Removes redundant null pointer checks checks

Removed 'optional' from proto3 syntax file.

--allow_oversize_protos=true is passed to bazel build.
When this macro is set, SetTotalBytesLimit is called to remove
the 64MB limit on binary protos when during ParseFromString.