Config server with settings

Added setting interface (ServerSettings and ServerInternalSettings) for server to access its configuration properties.

Enabled to obtain a server property with Config.get + property settings. Replaced the previous Apache Configuration + constant string property-accessing-pattern. 
IMO, this error message will not help users determine what to do to resolve this problem. I can tell that the semantics have changed here though, this is really a null check for the constructor argument now, right? I think we could remove this error message (assuming the same type of error message will be printed where the file is now loaded at, with instructions including the path the config file was expected to be located at), and with a null-check in the constructor of this class throwing nullpointerexception.