Prepare pantsbuild.pants 0.0.25 release.

Bump the version and add CHANGELOG entries.
Add a script to aid in prepping the CHANGELOG.

Testing Done:
Did a release dry run:
```console
$ ./build-support/bin/release.sh -n
```

Also rendered the CHANGELOG to sanity check - no warnings and looked good:
```console
$ source build-support/pants_dev_deps.venv/bin/activate
(pants_dev_deps.venv) $ rst2html.py src/python/pants/CHANGELOG.rst CHANGELOG.html
(pants_dev_deps.venv) $ open CHANGELOG.html
```

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/44395335

Bugs closed: 874

Reviewed at https://rbcommons.com/s/twitter/r/1496/
Sync of https://github.com/pantsbuild/pants/commit/8667470a7dd68b013ee9c8147ba9c7c19cd62ffe