Remove all reference to obsolete IConfigurationSectionHandler

[savornicesei]

See comment on PR #306

Changing the code to follow today's standards allows to get an instance of XslFilesSectionHandler returned by GetSection and as such access any code on it that could return a list of strings as expected by users of this section.

Note that there is another location where GetSection is called, inside CruiseServerFactory.CreateLocal, along with the ServerConfigurationHandler section handler that implements IConfigurationSectionHandler. I believe the same set of changes is required there as well.

There is one last usage of IConfigurationSectionHandler, and it's in CruiseControlConfigSectionHandler. But I don't understand its usage as its Create method returns null in all cases. Maybe it should just be removed.