Add package and publish support

Add package tasks for assets. Support adding a path prefix for Play dist.

Also support publishing the assets jar. This is disabled by default, and can be enabled in the same way as extra artifacts in other sbt configurations:

```scala
publishArtifact in Assets := true
```

By default publishing will be as a webjar, and in this case the packaging is different:

- Assets under `lib` are excluded. Webjars will be included as transitive dependencies. And otherwise there would also be duplicates in a nested lib structure -- for example, `lib/web-module/lib/jquery/...`.

- Remaining assets will be packaged webjar-style. For example, with a prefix like `META-INF/resources/webjars/web-module/0.1/...`.