Add basic i18n and l10n capabilities

Great library, and I might be using it in a web application soon. As such, I need to be able to internationalize and localize the app and the error strings that come with JSON-schema validation, if they are to be shown to the user. 

This pull request simply separates the currently hard-coded strings into a separate file, puts some gettext methods around them and allows for the use of custom strings through overriding the `ErrorString` class.