This reworks how we configure the `shadow` plugin in the build. The major
change is that we no longer bundle dependencies in the `compile` configuration,
instead we bundle dependencies in the new `bundle` configuration. This feels
more right because it is a little more "opt in" rather than "opt out" and the
name of the `bundle` configuration is a little more obvious.

As an neat side effect of this, the `runtimeElements` configuration used when
one project depends on another now contains exactly the dependencies needed
to run the project so you no longer need to reference projects that use the
shadow plugin like this:

```
testCompile project(path: ':client:rest-high-level', configuration: 'shadow')
```

You can instead use the much more normal:

```
testCompile "org.elasticsearch.client:elasticsearch-rest-high-level-client:${version}"
```