Log more information when purging directories

When you click the purge icon next to a directory purge on .../continuum/purgeConfigList.action, if there is nothing to purge, nothing gets logged.

Combined with the fact that nothing appears on the screen, you have no idea whether it worked or not.

I reduced the number of days to keep release directories, purged again, and finally saw some messages like:

2010-08-04 07:10:29,105 [pool-7-thread-1] INFO org.apache.continuum.purge.executor.DaysOldDirectoryPurgeExecutor - Purge All Directory Contents - releases-1276484258328

The purge process needs to log when it begins and ends (similar to the local repo purge) so you can tell what's happening.