Adds ImportMultiTask.

**Use case**

I pull down alot of different build.xml files with composer. I don't want to have to add multiple import references, especially if I am adding a new library.

**Solution**

Create a ImportMultiTask object for this case.

```
<import-multi files="vendor/previousnext/*/build*.xml" optional="true" />
```

**Help**

Can I please get some guidance on:

* If this could be committed to Phing.
* How I can get it over the line so I can be committed.