Enhancement: Support for keeping zipped dependencies in the assembled jar.

I've made changes locally to support keeping dependencies zipped and put them in a specified location within the jar (like onejar but without the classpath changing main class). This is useful for packaging builds for use on Hadoop which by default take dependencies from the 'lib' folder within the jar. It's an enhancement I think would be welcomed by Hadoop users.

Usage is by adding the following configuration lines to build.sbt

```
unzipDependency in assembly := false

dependencyPath in assembly := Some("lib")
```