def balance
      xml = Clockwork::XML::Balance.build( self )
      response = Clockwork::HTTP.post( Clockwork::API::BALANCE_URL, xml, @use_ssl )
      balance = Clockwork::XML::Balance.parse( response )
    end
def balance
      xml = Clockwork::XML::Balance.build( self )
      response = Clockwork::HTTP.post( Clockwork::API::BALANCE_URL, xml, @use_ssl )
      balance = Clockwork::XML::Balance.parse( response )
    end
Check the remaining credit for this account.
 @raise Clockwork::Error::Authentication - if API login details are incorrect
 @return [integer] Number of messages remaining