chore($translate): API connection + plain translations

Added an API connection. So now the module has to work exactly the same as before in case
of plain translations. Implemented a new API for fetching parts from the server. But, to
fetch some translation part from the server module requires an extended loader's API: it
has to handle a "part" option which represents a name of the needed part.

Breaking changes:
Module's API:
* "delPart" methods were removed
* "addPart" methods were renamed to "prefetch" and "fetch"