Dynamic loading of conf files specified by retrieved documents

Retrievers can now supply the `documentStore` with url-paths to additional (or overriding) configuration files. `state` will wait for the loading of those before the application is launched. This is done through a call to `configurator.loadAdditionalConf(confs)` which returns a promise, which resolves when all additional files are retrieved. (very nice API provided by `$q.all`!)

`state` currently doesn't care if these retrievals are successful - the promise is also resolved upon rejection. We might at least notify the user that something like this has happened in the future.

The `TreebankRetriever` already makes use of this - quite raw for now, the final markup to specify such configuration files will come with the new treebank schema. Right now a very simple link node suffices.

```xml
<link title="arethusaPluginName" href="urlToFile"/>
```

where arethusaPluginName could be something like `"relation"` with an href of `"http://services.perseids.org/arethusa-configs/arethusa.relation/elmlatch1.json"`.

The new repository for [arethusa-configs](http://github.com/latin-language-toolkit/arethusa-configs) holds some example conf files for relation labels. They are currently also available online at `services.perseids.org/arethusa-configs`.