Revert akka version back to 2.1.1, exclude akka in plugin-spark

1. Revert akka version in Shifu-core back to 2.1.1 and exclude akka in plugin-spark pom.xml
2. shifu - pom.xml: add plugin modules: shifu-plugin-spark,
shifu-plugin-mahout
3. shifu-core: 
	a) add ml.shifu.core.plugin.pmml package for the plugin interfaces.
	b) add ml.shifu.core.plugin.pmml.PMMLModelTest which is the abstract
test cases for all pmml model test cases.
4. shifu-plugin-*: update pom.xml and separate dependency