feat(contrib): add changelog script

This script assists in generating the changelog from one release
to another.

To retrieve the changelog from a specific tag to HEAD:

    $ ./generate-changelog.sh <tag>

To retrieve the changelog from two different tags:

    $ ./generate-changelog.sh <from> <to>
fix(CHANGELOG): prep for changelog script

This change is necessary to accomodate for generate-changelog.sh
available in contrib/util. With these changes, it's as easy as

    $ ./contrib/util/generate-changelog.sh v0.7.0 v0.8.0 | cat - CHANGELOG.md > tmp && mv tmp CHANGELOG.md

Which will generate the changes from v0.7.0 to v0.8.0 and append the
output to the top of CHANGELOG.md. The title was redundant, anyways.
style(contrib): make changelog consistent

The precedent is to generate the header of each version in
CHANGELOG.md as a h3 header. This bumps the header generation down
to that level for consistency.
docs(contrib): github-changes -> changelog script
Merge pull request #954 from deis/changelog-script

feat(contrib): add changelog generation script