Implemented configuration frame work

Added reading and writing machinery's configuration file.
The file machinery_config.rb contains the default values
used when no configuration file is present.

I think the default config file should rather be in `MachineryConfig`, not the base class.

I'd change the `Config` constructor to
```ruby
  def initialize(file)
    ...
  end
```

and add one to `MachineryConfig` like this
```ruby
  def initialize(file = Machinery::DEFAULT_CONFIG_FILE)
    super(file)
  end
```