Merge pull request msgpack#276 from msgpack/fix/rc

Prepare RC1 release

Fix net35 test build.

Update changelog.

Update change log.

Merge pull request msgpack#256 from msgpack/fix/msgpack#252

I confirmed branch CI log shows "build succeeded" but never ends...

Fix release date.

Merge pull request msgpack#250 from msgpack/wip/improve-packer-unpacker

Improve packer unpacker performance

Fix nuspec.

Add document to clarify msgpack#212.

Update version and changelog.