new feature: add white list function into spooling directory souce.

You can add white list into spool directory source by adding follows.
a1.sources.r1.spoolDir = /var/log/apache/flumeSpool
a1.sources.r1.whitelist = access

that means in dir /var/log/apache/flumeSpool, just the file name begin whit "access" could be read into flume, for example, "accessfile.log.2013-06-26" is a regular readable file.