Fix core for IDE >=1.5.3

This commit adds the missing key

```
attiny.upload.tool=arduino:avrdude
```

and merges the latest updates on analogPinToChannel().

Once merged you probably want to force ```ide-1.5.x``` branch to point to ```lib-1.5-cores```, and remove ```lib-1.5-cores``` afterwards since it's no more used.