load JAR from URL

doc/groovy/STIL.ipynb loads a JAR from a URL like this:
```
%classpath add mvn commons-io commons-io 2.6

import org.apache.commons.io.FileUtils

stilUrl = "http://www.star.bristol.ac.uk/~mbt/stil/stil.jar"
stilFile = System.getProperty("java.io.tmpdir") + "/stilFiles/stil.jar"
FileUtils.copyURLToFile(new URL(stilUrl), new File(stilFile));

%classpath add dynamic stilFile
```

would be cool to if the magic could do this directly, like
```
%classpath add url http://www.star.bristol.ac.uk/~mbt/stil/stil.jar
```