General Refactoring / Removing R18n

* Removed R18n. Now we are using just Active Support's `I18n` library.
* Removed the `function.rb` file. Now we have a directory called `config/initializers/` where we can setup everything Billbo needs. We also have a `helpers/` directory where we define some Sinatra helpers.
* Removed unused code and specs.
* Moved the bill related views to its own directory (`views/bills/`).
* Some other small improvements.

Please, review it and let me know if there is something you didn't like.