Dirisempty

As soon as this is merged in, it should be only a hop-skip-and-a-jump to complete delete. Fix for GlusterDirectoryIterator is in this commit: https://github.com/FIU-SCIS-Senior-Project-2014-Fall/Java-GlusterFS/commit/9ca094ff645fff0b9325f978d254b375111478a3
Tests for advance were added. One helper method is used for all of the cases, but while this limited code duplication, it may be more difficult to read. There is an alternative version that was written before this commit where the cases are separated (but has much more code duplication) in case you would prefer a greater focus on readability.