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.

- Kick off the conformance tests
- Add missing ignore for something generated by a build on the conformance directory.

Fixes issue #932.

Implement JSON parsing in C#.

Build protoc for host platform to enable cross-compilation.

Add support for POWER Linux

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

This includes all the well-known types except Any.
Some aspects are likely to require further work when the details of the JSON parsing expectations are hammered out in more detail. Some of these have "ignored" tests already.

Note that the choice *not* to use Json.NET was made for two reasons:
- Going from 0 dependencies to 1 dependency is a big hit, and there's not much benefit here
- Json.NET parses more leniently than we'd want; accommodating that would be nearly as much work as writing the tokenizer
This only really affects the JsonTokenizer, which could be replaced by Json.NET. The JsonParser code would be about the same length with Json.NET... but I wouldn't be as confident in it.

add warning notes for cc|py_proto_library rules.