Add locator service to arethusa.core
Change retriever arrays to objects in conf file
Let configurator.getServices return an obj
Set this.analyses in morph to {} on load

This is just for safety reasons - the application is not affected by
this, but without it it can happen that the console shows some errors,
when a user is selecting things too fast. A very minor problem.
Don't deregister the state.allLoaded watch

in MainCtrl. We will use this watch to trigger reloading of plugins
when we change e.g. from one treebank document to another.
Use locator service in SearchCtrl

Also rename the uri param to treebank instead of input for now.
Use locator service in state

... to retrieve uris for retriever plugins.
When no uri is given, do nothing to avoid hiccups.