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.

I've tested the new build file with the following commands:
ant clean
rm -f rain.jar workloads/*.jar
ant package
ant package-cassandra
ant package-cloudstoneNull
ant package-comrades
ant package-daytrader
ant package-gradit
ant package-hbase
ant package-httptest
ant package-mapreduce
ant package-mongodb
ant package-olio
ant package-raddit
ant package-redis
ant package-riak
ant package-rubis
ant package-s3
ant package-sample
ant package-scadr
ant package-scads


Best,

-- Marco