Standard abbreviations

Displays the standard abbreviation of authors and works, if possible.

If a `citeMapper` resource is configured, the `navigator` will try to look up cite information (provided by the `treebankRetriever`) by calling it. He passes the cite urn and ideally awaits a JSON response that contains the standard abbreviations for the urn.

`staging2.json` is using it already - the `citeMapper` is configured to use http://github.com/latin-language-toolkit/cite_mapper. Not a particulary good service, but it should work to get at least some good responses back.

The `treebankRetriever` is already looking for a `@cite` attribute in the treebank document. This is in accordance to the upcoming schema changes.