Add iteration capabilities to Config

Added #each method to be able to traverse each setting of a config
instance.
To provide this functionality was needed certain changes:

  * In order to be able to visit each setting as a key-value pair, was
    needed to retain the original hash in instance variable

  * Accessor methods that were generated dinamically now uses such hash
    instead of instance variables

Cleaned the interface of Config#build:
  * It was receiving an unused 'object' parameter
  * Original hash was passed as a parameter also, now it's accessed as
  an instance variable.
Merge pull request #3 from F-3r/add-iteration-capabilities

Add iteration capabilities to Config