Gradle 2388

Implemented [GRADLE-2388](http://issues.gradle.org/browse/GRADLE-2388) by adding the following tasks:

JavaPlugin:
* sourcesJar
* sourcesZip
* javadocJar
* javadocZip

GroovyPlugin:
* groovydocJar
* groovydocZip

ScaladocPlugin:
* scaladocJar
* scaladocZip

These are standalone tasks, not tied in with the lifecycle tasks by default.  As described in the forum idea these would most likely be used as follows:

```groovy
artifacts {
  archives sourcesJar, javadocJar
}
```

I'm sure there will need to be doc updates for this, but I wasn't sure where to start with those.

The other potential concern I have is that, as happened with some of the integration tests, this change could break the builds of people who have added sourcesJar/javadocJar tasks manually to their builds in lieu of a default provided by Gradle core.  On the other hand, it's an easy enough fix to just remove the tasks.