Disable rounding buttons when they cannot be used

```
Disable the rounding buttons when the cannot be used, for example: enter a
bill amount and then click round down.  Then disable the round down button
(because it already has been rounded down).

Or disable the round buttons when the total (or tip) amount is even (i.e.
you cannot round).
```

Original issue reported on code.google.com by `mand...@gmail.com` on 13 May 2010 at 8:26
