Proposition - Strings class' `pad` method

I have put the following (not really important) improvements:
- diamond operators everywhere,
- add `final` modifier on certain range of classes,
- in Strings default padding should be `PaddingType.RIGHT` - when `paddingType` is null- currently it's throwing NPE,
- remove unnecessary Autoboxing,

@lstypka please share your opinion on those - especially Strings class.