Added a mechanism to allow a plugin to import a package from org.eclipse...

In ManifestUtils.groovy, line 101, function `mergePackageList` there is this little snippet:

``` groovy
    /*
     * Here we fix the problem with eclipse 4.X bundles:
     * if 'org.eclipse.xxx' are imported via 'Import-Package',
     * the application throws ClassNotFoundException.
     */
    packages = packages.findAll { !it.key.startsWith('org.eclipse') }
```

I have a bundle that creates a custom 'WorkbenchRendererFactory', the element that turns E4 elements into SWT widgets.  For some reason, this bundle *has* to import some eclipse packages through the 'Import-Package' mechanism.  I've never understood how the dependency injection and classloading work together, I have to debug these issues by pure trial and error.

So for my "custom workbench renderer factory" plugin, I have the following line in my wuff configuration:

```
eclipseImports = 'org.eclipse.e4.ui.internal.workbench.swt,org.eclipse.e4.ui.workbench.swt.factories'
```

And now my build is 100% wuff!