Make `restrict: 'A'` explicit as Angular 1.3 changes default to 'EA'
Unregister previous listeners if changing data-l20n to an empty string
Remove the data-l10n-id attribute when changing data-l20n to an empty string
Move the localizeCurrentNode function outside of $observe

Otherwise removeEventListener wouldn't work correctly!

Also, tweak comments
Tag 0.2.1
Bump to 0.2.2-pre
Fix linters configuration
WIP