extraJvmArgs property for GWT task

Hello!

I added a new property extraJvmArgs for GWT task (as in GWT maven plugin). This value will be tokenized and passed to forked java process for GWT compiler as additional arguments.

```
gwt {
    gwtVersion="2.6.1";
    minHeapSize = "512M";
    maxHeapSize = "1024M";

    extraJvmArgs="-Djava.io.tmpdir=/run/shm"

    modules = ['org.example.MyModule']
    src = files('src/main/java', 'build/resources/main')
}
```