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

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.

Bazel can build protobuf when it's not in the root

That is, Bazel can now build protobuf when the latter resides in a subdirectory of a project.

This eliminates an unnecessary reload of buffer_ that occurs (before
this change) after the store to *value.

Removes trailing whitespaces

Fixes #2669.