Translation via angular-translate

New version of translation mechanism via angular-translate.

What's new:
- language switch
- language fallback (en-us is default)
- more cleaner code for some translations
- tables header translation
- translation of statuses and severity 
- translation of pop-up windows
- usage of $translate service was excluded
- etc

TBD:
- translation in graphs ('translation' directive isn't working in \<svg\> area)
- translation of some text in JS code (another realisation is needed)
- localisation of date/time (switching of angularjs locales scripts)
At first, we are using camel-case notation, please use `selectedLang`, not `selected_lang `
Then I propose you more elegant solution for language change: 
```js
$scope.selectedLang = $scope.langs[0];
$translate.use($scope.selectedLang.locale);
```