Reorganization of build.xml and JAR files

I've performed changes to the build.xml in order to improve modularization and reduce the size of JAR files.
With this restructuting, now the rain.jar file only contains core classes (i.e., the ones that are workload-unaware).
The workload-aware classes, along with their third-party dependencies, are confined to the related workload JAR file.
For instance, the JARs in thirdparty/mongodb-2.5.3 files are only compiled for the mongodb workload and only included in the workload/mongodb.jar
This also should improve maintanability and dependency problem since two or more workloads are not forced to use the same version of a given third-party library.
In addition, this should speed-up compilation of the core classes and of workload with few dependencies.

Please, consider to merge this changes.

Best,

-- Marco