Various changes and bug fixes

-Added JAR hashing to prevent multiple copies of jar files in the database
-Added a node health status reporter (cpu, memory, disk space)
-Fixed a bug that prevented jar removal messages from being sent to workers
-Added a setting to control the progress update interval on workers
-XML files are now parsed on the client before the job upload process starts
-The upload no longer crashes when it receives invalid xml files
-Fixed an exception reporting bug in the worker

Signed-off-by: pleiades <pleiades@pleiades1.cs.up.ac.za>
Merge pull request #46 from benniel/changes

Various changes and bug fixes