add plural helper for i18n

[schinken]

Now it's possible to use pluralization using messageformat

• https://github.com/SlexAxton/messageformat.js#pluralformat
• http://angular-translate.github.io/docs/#/guide/14_pluralization

[schinken]

One last note: The messageformat usage has drawbacks! (http://angular-translate.github.io/docs/#/guide/14_pluralization#pluralization_the-drawback)

If we want to use angular features inside our translation-strings, we need remove useMessageFormatInterpolation() and use addInterpolation('$translateMessageFormatInterpolation'). But in this case, we have to adjust our translate calls for snippets which uses messageformat