implemented size calculations.

still not sure if total/free/used would be better than this.

Unit test sizes have been checked aganst Unix ```df```:
```
/dev/loop0          1004         6       998   1% /media/jan/8716-FC70
/dev/loop0          9962         6      9956   1% /media/jan/B384-1B23
/dev/loop0         39368         2     39366   1% /media/jan/testdisk
```