fixes #646

It turns out that imho when talking about application groups (identified
by name) and application instances it's just logical to speak of
instances and applications. So the Application class get's renamed to
Instance. So an Application is now a set of Instances with the same name

With this change it is possible to include credentials in the instance
metadata which will then be used to access the client endpoints using
HTTP Basic authorization.

closes #359

* Separate AppStore from AppRegistry
* Allow Id generation to be altered
* Remove logic from Application; Move id genretation into AppRegistry
* Make AppStore threadSafe