Fix #751: stdout/stderror log appenders are broken. To achieve this properly, I refactored AgaviLoggerAppender to be an AgaviParameterHolder (closes #774) and introduced an intermediate AgaviStreamLoggerAppender to abstract things properly (closes #773). Just for the record, the error only appeared on Linux ;) so we now use "w" instead of "a" flags for std* fopen() calls.