Support localization throughout the code

This is the changes we talked about in pull #89. It adds localization support to the different conversion functions.

Maybe I should just remove the deprecated functionality now as the next bump is 2.0 as you said. But I don't know – you can check it out in [deprecated_localization.rb](https://github.com/lassebunk/stringex/blob/localized_conversions/lib/stringex/deprecated_localization.rb).

:-) /Lasse