Make GetAndFlatten public

This allows us to tweak the accepts header and build up
our own requests without duplicating this logic.
Merge pull request #544 from octokit/haacked/make-get-and-flatten-public

Make GetAndFlatten public
Add release notes for 0.4.1
Merge pull request #545 from octokit/release

Add release notes for 0.4.1
Fix typo in release notes