Updated upb and simplified ruby code a bit with new upb method.

Proper checking of enum with non zero default

proto2 syntax allows the first enum to have a non zero value. This means any
field using that default has a non zero default without having an explicit
default being set. So when deciding what runtime info is needed, don't rely
on an explicit default, always check that the values aren't zero.

Fixes https://github.com/google/protobuf/issues/1453

Added serialVersionUID to ExtendableMessage.

* Add initial design doc for Swift protocol buffers.

Add a docs directory and move the third-party add-ons page here.

Change protobuf CPP proto generator to support the 'lite' option in

Remove googletest.h header from stringprintf.cc

Regenerate descriptor/well-known types for C#

(There are documentation changes and new fields in descriptor.proto that have resulted
in changes to the serialized descriptor, but no breaking changes for C#.)

ObjC support for failing the build in the generated WKTs are out of date

- Always generated into a temp directory so we can see if things changed.
- Add a flag to control exiting with error when stale vs updating.

This should let the continuous builds error out when ObjC needs to have the
checked in sources updated.

Enum casing in C#

Support Windows in gulpfile.js

(JSON parsing already does the right thing.)

JSON tests fail, as we're not using OriginalNameAttribute yet.

Overview of changes:
- A new C#-specific command-line option, legacy_enum_values to revert to the old behavior
- When legacy_enum_values isn't specified, we strip the enum name as a prefix, and PascalCase the value name
- A new attribute within the C# code so that we can always tell the original in-proto name

Regenerating the C# code with legacy_enum_values leads to code which still compiles and works - but
there's more still to do.

Add missing includes in field mask test

Fix comments typo and update WKTs

Add missing PY2AND3 srcs_versions attributes to Python Bazel build targets

Update file list to include the missing extension lite file.

Updated upb: JSON parser now accepts both camelCase and original case, and flag to choose on output.

proto3.

Added a couple unit test proto3 files, for arena_lite and lite. Cloned
the proto3_arena_unittest to test some of the basics of generated code
(and to ensure that the generated proto3 test files are used by some
test).

For JSON encoding we provide a new option to decide at
encode time whether to use camelCase or original proto field
names:

  json = MapMessage.encode_json(m, :preserve_proto_fieldnames => true)

It doesn't seem to be necessary here, and it pulls other testing headers
during compilation of release protobuf.

The flags are:

  UPB_JSON_ACCEPT_LEGACY_FIELD_NAMES
  UPB_JSON_WRITE_LEGACY_FIELD_NAMES

The first just allows the parser to accept the old field names.
The second makes the printer print the old field names.

These flags are intended to be temporary, as a migration aid
for users.

Internal merge