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.
Thinking again, the default configs are indeed special, because they also define which configuration options exist at all. So maybe it actually would be appropriate to do it in Ruby. We also need to add a description of the configuration options somewhere. This file would be the natural place for it. Then a plain hash would not be the best format, though.

Maybe something like:

```ruby
add_config("hints", true, "Show hints about usage of Machinery in the context of the commands ran by the user")
```