Support "-" as delimiter in language codes

[agrajaghh]

Currently the fallback language is determined form the language code using the underscore ("_") as a delimiter. At least Chrome is using a dash ("-") for me, so gettextFallbackLanguage() is returning null for e.g. "de-DE" instead of "de".

see: https://github.com/rubenv/angular-gettext/blob/master/dist/angular-gettext.js#L547

[atapas]

+1