Change target name from 'default' to 'publishPlugin' to allow target to be called from other scripts

I am trying to integrate the grails-release plugin into an automated CI workflow, and the 'default'; name for a target appears to not work when attempting to make use of PublishPlugin.groovy from another script.

Rather than loading the script manually as a File object, or executing a command-line expression, changing the target name to something accessible seems to be a cleaner solution.

With this change, I can use:

```
    includeTargets << new File("$releasePluginDir/scripts/PublishPlugin.groovy")
```

and 

```
    depends(publishPlugin)
```

to include it in my workflow (with appropriate script arguments).