Fix clearing wrapper type fields with reflection.

The nullable value type fields already worked, but the use of the CLR property concealed the difference between string and StringWrapper fields.

Get VS 2015 to use const int definitions

VC++ up to VS 2015 RTM does not require explicit storage allocation for
static const integers declared in classes. VS 2015 Update 1 requires
these storage definitions in some cases. It's unclear exactly what
cases - simple tests work with and without the explicit storage
allocation.

Many previous versions of VC++ have theoretically *allowed* a
definition to supply storage, but tests on VC++ 2013 show that this
doesn't actually work correctly - it leads to duplicate definition
errors in Chromium. So, the change is scoped to VS 2015 only.

This change also updates the generated files to match the new generator.

TL;DR - this change is necessary in order for Chromium to build with
VS 2015 Update 1.

Removed all warnings from the Python/C++ build

Also made the Travis build ensure that no warnings
are present.

These builds were previously spewing many warnings,
which was hiding warnings for important things like
accidentally using C++11-only features.

Change-Id: I56caeee9db48bc78756a3e8d7c14874630627037

add default_runtime attribute to cc|py rules.

This adds implicit dependencies to the coressponding runtime libraries.

Use oneof case in equality tests

The included C# test will fail until the regenerated code is used, which is in the next commit.

Change the #include for any.h to use angle brackets instead of quotes

Change the #include for any.h to use angle brackets instead of quotes, to be consistent with other protobuf library includes.

Enable python bazel tests and explicit list python dependencies.

--strip_prefix doesn't seem to work.

Add runfiles to make the protobuf_test pass.

Rename python and java bazel rules.

Use && for internal_copied_filegroup.

So that the rule fails if one or more files cannot be copied.

Python bazel support

We need to use the list to indicate field presense.
The field must only contain 0 or 1 string element.

fix json_format for python2.6:

1, objcect_pair_hook is not supported in python2.6, so duplicated key check is removed in 2.6
2, total_seconds is not suppoted in python2.6, changed to compute seconds directly