Use standard I18n for currency and dates

My main goal was to add year to printed dates, but I ended up with this.
Rename purchased_at -> purchased_on

The column is a `Date`, so it should have the `_on` suffix. The `_at` suffix is
reserved for `DateTime`s.
Merge pull request #114 from datasektionen/vk-timestamp-year

I18n for currency and dates, and rename purchased_at -> purchased_on
Rename purchased_at -> purchased_on in tests too
Remove .gitkeep files
Uninstall Guard

It's not used anymore.
Remove some empty files
Uninstall Capistrano and remove related files

Cashflow is currently deployed via Git (à la Heroku).
Auto-format code using RubyCop 0.28.0

Command used: `rubocop --rails --auto-correct`

Also add some RuboCop configuration and manual corrections.
Batch trim trailing whitespace
Fix some errors and warnings

* uninitialized constant BusinessUnitsHelper (NameError)
* superclass mismatch for class Mysql2Adapter (TypeError)
* Passing a template handler in the template name is deprecated. ...
Fix failing spec

Error was

> ActiveRecord::DeleteRestrictionError:
>       Cannot delete record because of dependent purchases

Remove the block and depend in DatabaseCleaner to clean the database properly.
Kill off XML responses

We don't use these.