Fetch release notes via the releases API

Squirrel for Mac sends down the latest release notes but Squirrel for Windows does not so another solution is needed to show the release notes in app.

This PR uses the GitHub releases API directly for the atom/atom repo to pull down and render the release notes.

The notes for the last 30 releases are shown instead of just the last one giving more context for people jumping multiple releases on an upgrade.

Closes #16
Closes atom/atom#3757
Refs atom/atom#4244