DSL support

Support for DSL files associated to DRL files. This changes doesn't take into account the possibility of different DSL for each DRL included in the ruleBase. I doubt that is a very common issue though, but if you think there is value in that let me know. I also upgraded slf4j to 1.7.0, but anything after 1.7 should be fine if you have something else in mind.

Thanks a lot for your work!