generate changelog via mdchangelog spumko/hapi#1606

As requested at https://github.com/spumko/hapi/issues/1606#issuecomment-41769063, changelog output generated from [mdchangelog](https://github.com/creativelive/mdchangelog) command:

```
mdchangelog 19a6179...beb3d50
```

If this changelog generation is suitable, it could be incorporated into a build process (for major releases). If `mdchangelog` put the last sha at the top of the file (which it doesn't currently do), maybe something like:

```
mdchangelog HEAD...`head -1 CHANGELOG.md` >> CHANGELOG.md
```