JSMessages: add support for messages containing plural forms
JSMessages: removing typo in unit test
JSMessages: polishing the comments
JSMessages: polishing the comments
JSMessages: polishing the comments
JSMessages: adding a note about language support
JSMessages: 'plural' does not have to be capitalized
JSMessages plurals: in case of missing or invalid plural arguments, maintain the MW behavior compatibility
JSMessages plurals: when there is not rule for a language sub-code, use the main language rule as a fallback
JSMessages: tiny fix in the code comment
JSMessages: small code simplification
Merge pull request #1492 from Wikia/jsmessages-plurals

JSMessages: add support for messages containing plural forms