Implemented settings frame work.

Added class Settings which handles reading and writing machinery's
configuration file. The file default_settings.rb contains the
default values used when no configuration file is present.
It would be better, if the default settings are not done in Ruby code, but in a config file itself. This way we only have one way to express configuration, and the mechanism to overlay configuration from a default file and a local one could be easily extended to also include for example a system wide configuration file installed by the user.