feat(contrib): add changelog generation 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>

And - le pièce de résistance - automagically generate the changes from one release to the next:

    $ ./contrib/util/generate-changelog.sh v0.7.0 v0.8.0 | cat - CHANGELOG.md > tmp && mv tmp CHANGELOG.md

This will generate the following:
```
><> ./generate-changelog.sh v0.8.0
### v0.8.0 -> HEAD

#### Features

2bce3f4 feat(contrib): add changelog script
4712658 feat(contrib): add CoreOS flair to motd
11b07e2 feat(contrib/coreos/user-data): create Deis motd
4808ea9 feat(client): add --app option to apps:run
b2e2b78 feat(client): add auth command

#### Fixes

166282a fix(CHANGELOG): prep for changelog script
48cd94f fix(client): return 1 on two more error cases
45e5e0c fix(contrib): correct coloring
23f0f0c fix(logger): allow digits in app name when parsing log file
33f72a1 fix(controller): restore App.destroy() and call delete()
998498f fix(Makefile): fix 'check-fleet' rule for $FLEETCTL_TUNNEL with port.
6d64cbf fix(readme): unify language with deis.io
c510fc5 fix(controller): give processes unique counters
2c699ed fix(controller): remove username length validation
75aeb67 fix(registry): store apps as appname
926c8f4 fix(client): return non-zero on errors
a0cf80c fix(docker): use btrfs inside builder
15751ad fix(builder): do not check for a Procfile
4edfb2c fix(pip): update location of pip installer

#### Documentation

900e583 docs(contributing): add TESTING footer
842838a docs(vagrant): update references to vagrant files
2dd299c docs(README): install nfs on Ubuntu
98b478a docs(contributing): split out DCO

```