Unified writer

[tonystone]

Description

Added the UnifiedLogWriter, which writes to the Apple Unified Logging system. This is only available on Apple products.

Motivation and Context

Give users of TraceLog for Apple Platforms the ability to write to the Apple Unified Logging system without having also to include the Adaptive Writer project

How Has This Been Tested?

It's been thoroughly tested in use in several applications by QA.

Checklist:

For all changes:

• Is there an issue associated with this PR? Is it referenced
  in the commit message?
• Has your PR been rebased against the latest commit within the target branch (typically master)?
• Is your initial contribution a single, squashed commit?

For code changes:

• Avoid other runtime dependencies
• Have you written or updated unit tests to verify your changes?
• Have you ensured that the full suite of tests is executed via make tests in the cmake-build-debug` directory off the root of the project?
• If applicable, have you updated the documentation?
• If applicable, have you updated the CHANGELOG.md file?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[marinaandricic]

Looks good