Used Uint8List instead of List<int> for performance improvement.

The method is called `fromBytes` so it's likely that the parameter passed will be of type `Uint8List`. Now the `List.setRange()` implementation will copy the list element-wise, while `Uint8List.setRange()`, when passed another `Uint8List`, will use native code optimized to copy arbitrarily sized chunks of bytes.
Merge pull request #31 from stevenroose/small-patch-1

Used Uint8List instead of List<int> for performance improvement.