first attempt at stable packages.

you can build stable versions with,

```
make STABLE=t recipes/evil
```

this is a rough cut and by default only supports git recipes. by default it checks all tags and will pick the tag that is the highest emacs version.

* evil
* magit
* hl-sentence
* powerline

are examples of packages that work already---as far as I can see

stable builds end up in `packages-stable`

:shipit: