chore(changelog): add commit-msg hook to verify commit messages

This adds a Git commit-msg hook which runs after a commit message is
written. It verifies that is matches a certain spec as defined by the
AngularJS document here:
https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#
This will allow for autmoated generation of Changelog.md in the future.

Migration notes: Please run `ln -sf ../../validate-commit-msg.js
.git/hooks/commit-msg` to install the commit hook.
chore(changelog): add git-changelog config and sample CHANGELOG.md

This is the setup for the Grunt task that generates a changelog. Note
that it currently generates the changelog for the range
changelog-start..HEAD where changelog-start is a tag. It will always use
the most recent tag it can find. We may want to change this
behavior. Also, the original repo has some bugs, so I'm using my fork
until he accepts my PRs.

Migration notes: To run: `grunt git_changelog`
Merge pull request #2424 from colegleason/changelog

chore(changelog): add commit-msg hook to verify commit messages