Need better way to set version on a Puzzle()

when initializing a `Puzzle` object (not loading from a file) if you want to use a version other than 1.3 you have to manually set two properties

```
puz = Puzzle()
puz.version = '2.0'
puz.fileversion = '2.0\0'
```

Should provide a better way either by generating `fileversion` from `version` and/or with a `version` keyword arg to the `Puzzle` initializer.