Music Import Bug

When importing music, the program crashed with:

```
java.lang.IllegalArgumentException: URI is not absolute
    at java.io.File.<init>(File.java:416)
    at grow.SaveManager.saveSound(SaveManager.java:638)
    at grow.SaveManager$7.act(SaveManager.java:841)
    at grow.GrowGame.doTurn(GrowGame.java:222)
    at gui.Grow$GameThread.run(Grow.java:665)
```

OS X 10.11.4, Java 1.8.0_74
