Add support for pure ActionScript test runner

Hi,
I'm using the default flex unit runner generated by FlashBuilder. It's called FlexUnitApplication.as and how you can see, it's written in pure ActionScript, no Flex. 

If I try:

```
flexUnit {
    template = 'src/FlexUnitApplication.as'
}
```

build fails with following error:

```
build/gradle/build/reports/FlexUnitRunner.mxml(1): col: 1 Error: Content is not allowed in prolog.
  
  package
  ^
```

The problem is that gradlefx copies the content of the .as file into the .mxml file.