small documentation fix
+ new api command AccbalGet to get account balance
+ accbal spec
+ poor man's logging
* some more error context
 Respect collmex api (which differs from documentation http://www.collmex.de/cgi-bin/cgi.exe?1005,1,help,api_Salden)
Fix spec to respect changed error message.
Merge pull request #7 from iaddict/new-api-accbal_get

New api AccbalGet