Support for disabling log writes on shutdown.

Updated logger.js to support disabling all log writes.
Updated log4js.js shutdown function to disable log writes.
Added tests.
Update gitignore to ignore rolling date stream's test output.

.gitignore

@@ -4,3 +4,4 @@ build
 node_modules
 .bob/
 test/streams/test-rolling-file-stream*
+test/streams/test-rolling-stream-with-existing-files*