Lets do something about that D score that double_entry.rb received

- Slice off the `balance` method guts and objectify it
- move errors into their own file
- slice off `account` method guts
- slice off `transfer` method guts
- move the `reconciled?` method to the `Reporting` module.

https://codeclimate.com/github/envato/double_entry/compare/refactor-doublee-entry-balance-for-less-d