Fix connector server changes for async getTranslators()
Make translator loading asynchronous

Other changes:
- Factored out Zotero.Translators from Zotero.Translator. The latter
  should be usable in the bookmarklet and connectors without changes.
- configOptions, displayOptions, and hiddenPrefs no longer copy on
  read. I don't think this actually affects any existing code.
- Zotero.Translate._loadTranslator() now returns a promise