Support for overdraft in checking accounts

[pashagolub]

Most of my debit cards have overdraft enabled, means value can be negative up to certain amount.

At the moment paisa has two issues working with such accounts:

• Doctor page displays warning

The running balance of an asset account is not supposed to go negative at any time. This issue typically happens due to incorrect transaction entries.

• Balance Units are not displayed on the Balance page. On screenshot below you can see only negative Market Value without Balance Units. However the next one with positive amount is OK.
  

[ananthakumaran]

I need to think about how to support overdrafts. Balance units is shown only if you hold commodities and the balance is non zero.