Added PatternLayout for intended multiline logging.

Added Log::Log4perl::Layout::PatternLayout::MultilineIndented and a test for it 
to have a layout for logging this way :

    2013/07/30 21:57:42 > This is
                          a message with
                          multiple lines
    
    
Sorry, second and third line of log message should be indented properly. Do not know how to do for a comment.
Edit: Got it ;-)