Notes about this commit: - this feature comes in replacement of the previous i18n system, which made compiling Opa applications much harder - intl is based on Format.js' intl-messageformat library and its ICU message formatting options. - the syntax is extended with the new directives @intl and @locale which, respectively, build a new translated string, and return the client locale selection. - the syntax of strings inside @intl directives has been modified to be able to parse ICU message formats appropriatly. - Finally, IntlMessageFormat does not translate base strings. Translations files must be included in a predefined locale directory.