Add build of jars with classifiers

Adds a :classifiers project key that takes a map from classifier keyword to profile map.

```clj
:classifiers {:tests {:source-paths ^:replace ["test"]
                              :resource-paths ^:replace []}}
```

Depends on https://github.com/technomancy/leiningen/pull/991 and https://github.com/cemerick/pomegranate/issues/52