check if the element exists
refactor bindings to use localize, fire DocumentLocalized once, and LocalizationReady on each retranslate
remove node.retranslate() for now, since we don't have it working with responsive
Bug 864824 - Context should be able to switch to a new set of locales. r=gandalf

For the use case of the user switching the language, there should be a way to
change the locales available in the context.

 - Rename GlobalsMnager to RetranslationManager
 - Relax the requirements for calling addResource & linkResource

   The summary of the (add|link)Resource logic is as follows (`y` means "the
   method adds resources to this locale or these locales"):

                                 _none   _locales
        addResource                y
        linkResource(string)       y         y
        linkResource(function)               y

 - Make get and getEntity synchronous only
 - Context.localize() should return an object, for future-compatibility
 - Context.registerLocales() may call freeze() if the context is frozen and
   ready;  you don't need to call freeze yourself.