Basic lib for saving debts
!iou command saves debt
Store total debt value

Use total as accessor, which is populated at creation
List debts to a certain user
Replaced Debt#amount with Debt#value
Added money value
Subtract amount from the opposite debt
Refactored Debt class

- Namespace
- One key in the database
- Sets the amount depending on who is the sender in the key
- Extracted key logic into it's own class
Spec fixes
Debt#previous_value is public method

Debt#create! returns true or false
Fix !iou command to new debt class

- Rename class to IOweYou to avoid name collision
More pretty output from !iou

- !iou without receiver returns usage instructions
Auto-require the iou libs
Tidied up the IOU::Debt code
Fixed faulty debt value
Refactored IOU::Debt class

- Renamed Debt#value to Debt#balance
- Rename balance method