There are corner cases where MessageDescriptor.{ClrType,Parser} will return null, and these are now documented. However, normally they *should* be implemented, even for descriptors of for dynamic messages. Ditto FieldDescriptor.Accessor.
We'll still need a fair amount of work to implement dynamic messages, but this change means that the public API will be remain intact.

Additionally, this change starts making use of C# 6 features in the files that it touches. This is far from exhaustive, and later PRs will have more.

Generated code changes coming in the next commit.

Added the type registry in advance of implementing Any support.

Biting off just this bit first as I don't need the changes from a previous PR for this part.

Fixed a bug in CSharp SampleUsage.

Rename "umbrella" to "reflection" consistently.

add headers

This changes csharp_names.h, which will require a corresponding change in GRPC.

Parser property in MessageDescriptor

Generated code coming in next commit - in a subsequent PR I want to do a bit of renaming and redocumenting around this, in anticipation of DynamicMessage.

Change the way the "umbrella class" (descriptors) is named/namespaced

Reorder the checks so anything in the expected file is an implicit whitelisting

In the old flow, any 2 char prefix in the expected file was still generating a
warning about being a poor prefix. Now we check the expected file first, so
anything expected is let through.

Make the cc wkt proto target public.

Cleanups of deps and ignores for conformance

- Hopefully complete the deps for other languages for the generated conformance proto sources.
- List the generated sources for cleanup by make's clean rules.
- Make the toplevel nuke the pyc files that can get created in the ObjC dir.

Switch ObjC Travis testing to using xctool

xctool is preinstalled on the Travis OS X images and it seems to do better with
iOS simulator flake, so use it instead of xcodebuild.

xctool also is less chatty compared to xcodebuild, so it makes the logs a little
easier to read.

Add ObjC build to travis setup

- Add objc for iOS and OS X builds to travis configs.
- Update handing of python install to deal with newer OS X versions.

Instead of having a Proto nested namespace to avoid conflicts between the descriptor-holding static class and message classes, just append "Reflection" to the name.
Generated code changes (and corresponding manual changes) in following commit.

Stop removing all blank lines in doc comments.

Tweaks to the Mac build script

This fixes issue #832.

Generated code changes in next commit.

Add recursion limit handling to JSON parsing.

include plugin.proto and generated code in protobuf-java

- Support building with Xcode 6.4 or 7.x
- Fix an error in usage info.
- Add a flag to build the core parts of protobuf only.

Added a TODO around a possible change to the tokenizer API, changing PushBack(token) into just Rewind() or something similar.

Move the creation of the "fields by JSON name" dictionary to the descriptor

Created a new exception for JSON failures.

Update the Mac build script to include the conformance tests

This is only thrown directly by JsonTokenizer, but surfaces from JsonParser as well. I've added doc comments to hopefully make everything clear.

The exception is actually thrown by the reader within JsonTokenizer, in anticipation of keeping track of the location within the document, but that change is not within this PR.