Add file transfer task

I'm not entirely happy with the API atm, I'd have loved to have a configuration closure instead, something like:
```
command {
  ...
  push {
    localFile 'img.jpg'
    remoteFile 'sdcard/Pictures/img.jpg
  }
}
```
but I'm not sure how to implement this. If anyone can give me any inputs on how to achieve it, I'd be happy to put more work on it.