Fix unresolved symbols with MSVC12 and /Zc:inline

Speed up JSON parsing.

const FieldDescriptorCompare

Fix unused variables warnings in generated_message_reflection.h

README.md: Make docs URL a link

FreeBSD compatibility

Add GPBMessageDropUnknownFieldsRecursively() and tests.

GPBMessageDropUnknownFieldsRecursively() is a new helper to drop the
unknownFields from a message and all sub messages (in fields or extensions).

- Adding more detail on running tests
- Adding info about the status of .NET 3.5 support and how to enable .NET 3.5

NUnit 3.4.0 —> 3.6.0
dotnet-test-nunit 3.4.0-alpha-2 —> 3.4.0-beta-3

* Changing DOTNET35 framework symbols in preprocessor directives to the default built-in value of NET35.
* Adding extension method StreamExtension.CopyTo for .NET 3.5 because it didn’t exist until .NET 4, and adding associated unit tests.

Add an option to always print enums as ints in Json API

https://github.com/google/protobuf/issues/2735

Dedent URL so it renders as a link and not as code (fixed width font and not clickable).

Clang now validates that <set> comparators must have a const operator():
https://reviews.llvm.org/rL291969

Discussion:
https://groups.google.com/d/msg/protobuf/9W6zFIHaJ-4/9RrfwelpEQAJ

In #726 and #813, linking errors with MSVC14 were resolved. The change in
MSVC12 leading to these errors was not a newly introduced bug, but that
/Zc:inline was made on by default. This option is also available with MSVC12,
so the workaround should be applied for it too.

Merge #2653, #2622, #2579, #2652

This allows more compact comparisons (1 byte instead of 4 byte
immediates on x86) for each possible wire/tag inside each field.

rvalue setters for scalar string fields were added in #2506.

Inline branch-less VarintSize32/VarintSize64 implementations.

Avoid aliasing CodedInputStream::buffer_ when parsing little endian integers

--[name]_opt support depended on the plugin being register, and didn't support
working with just --[name]_out directive (where the plugin is found via the
users PATH. This extends the command line handing to allow --[name]_out to
be all it takes for the _opt directive to also be supported.

Fixes https://github.com/google/protobuf/issues/2712

Undef TYPE_BOOL to avoid conflict with iOS.

Add csharp compatibility tests against v3.0.0

JsonUtilTest: Add ParsePrimitiveMapIn subtest

Fix compiler warnings about unused variables in wire_format.h

the deprecated atomics.

Add bazel protobuf resources

All the files are copied from 3.0.0 (JosnFormaterTest was deleted)

Fix python compatibility test

Fix python compatibility test when a new generated code imports an old version(2.6.1 or older) generated code.

Update load() statement to latest style

The first argument is currently implicitly a .bzl file.
Change this to be explicit.