Addressing issues for dartbug.com/6442. Still the new sprint stuff seems...

... to be broken. When you get a chance can you fix and merge, this package will not be updated on pub until following is satisfied  http://code.google.com/p/dart/issues/detail?id=6442


Some exceptions with sprintf.
```
NoSuchMethodError : method not found: 'call'
Receiver: 0
Arguments: []
#0      PrintFormat.call (package:sprintf/src/sprintf_impl.dart:92:53)
#1      sprintf (package:sprintf/sprintf.dart:16:23)
#2      LoggerImpl._format (file:///Users/adam/dart/log4dart/lib/src/logger_impl.dart:82:54)
#3      LoggerImpl.errorFormat (file:///Users/adam/dart/log4dart/lib/src/logger_impl.dart:41:36)
#4      SimpleLogTest.SimpleLogTest (file:///Users/adam/dart/log4dart/test/simple_log_test.dart:16:24)
#5      main (file:///Users/adam/dart/log4dart/test/test_runner.dart:27:3)
```