Require VS2015 in the solution file

Add atomics support for 32-bit PPC.

Added support for CommonJS require()

Support ObjC Generic Collections

Validate that end-group tags match their corresponding start-group tags

Some fixes for the most recent release notes.

- Extend GPB*ObjectDictionary to support generic syntax.
- Update the generator to output generics so the enclosed type is exposed for compiler checks.
- Use generics in a the public interfaces.
- Update the generated sources that are checked in.

"nodejs" does not exist on Travis, it appears.

List python_cpp as failing on OS X

Disable the xctool updates

In light of https://github.com/google/protobuf/issues/1232, disable the xctool
update so we stay on 0.2.7 which seems to work better.

Remove the passing of -newSimulatorInstance since the older xctool doesn't
support it.

Generate C# code whenever descriptor.proto changes

Added ALLOW_UNLIMITED_BINARY_PROTO macro and setting it when

Add more tests around merging wrappers

Removes redundant null pointer checks checks

Removed 'optional' from proto3 syntax file.

--allow_oversize_protos=true is passed to bazel build.
When this macro is set, SetTotalBytesLimit is called to remove
the 64MB limit on binary protos when during ParseFromString.

Move the xctool use of -reporter into a common spot and always use "plain" to get more readable logs on travis.

Move the xctool use of -reporter into a common spot and always use "plain" to get more readable logs on travis.

Tweak the xctool run for iOS tests to try and sort out flake

- Use the plain reporter for slightly more readable logs.
- Add -newSimulatorInstance to the run-tests to see if it helps with flake.

Bump up travis to Xcode 7.2

- Update simulator versions used.
- Mark the iOS tests as flaky while trying to dig out the root cause.

This was in an attempt to fix the wrapper handling corner case, but it's really fiddly.

This detects:
- An end-group tag with the wrong field number (doesn't match the start-group field)
- An end-group tag with no preceding start-group tag

Fixes issue #688.