doc: Merge 1.8.x missing changelog entries.

The current changelog completely misses v1.8.2, v1.8.3 and v1.8.4 releases, this patch cherry-picks the relevant release commits. Only CHANGELOG.md is updated.

These are now ordered by version, not by date. I could reorder them by date, if that's preferred. Also, it's done in three separate cherry-picked commits by @rvagg and @Fishrock123.

Edit: ordered by date, references to iojs/io.js (and other forms) were replaced with nodejs/node.