Make ModulePath configurable via the application/web configuration file.

See issue #86.

Add modulePath="" attribute to kentor.authServices configuration. If not specified, defaults to "~/AuthServices".

With this change, I believe that the assertionConsumerServiceUrl parameter can be removed and generated dynamically. Although I don't see the need, I suppose we could leave the assertionConsumerServiceUrl  attribute and just dynamically generate the value from the modulePath value if it wasn't configured or explicitly defined. A consumer could always override the default.

Taking it to the next step, the /acs portion of the Url could also be generated by the CommandFactory dictionary. Same could apply to the /signup and forthcoming /metadata commands.
This line is not entirely covered by the unit tests. To provide a default value it is better to use `DefaultValue=` of the `ConfigurationProperty` attribute.