Add dynamic properties to a config file

The service has to be configurable and all properties that are changing
from deployment to deployment should be outsourced to a config file.
Create a basic implementation of this config file and load it in the
service main to check if it's working.
Adapting the Settings class

Create a settings class that is acting like a singleton and prodives
access to values in the app.config file. This file should be able to
handle settings in the Rhenus.Service config group only, leaving the
settings in other settings groups to the implementing classes of
modules. This way, the config file can contain the settings for all
modules in separate groups.
Merge pull request #3 from textquell/feature/dynamicsettings

Introducing dynamic settings