1. shifu - pom.xml: add plugin modules: shifu-plugin-spark,
shifu-plugin-mahout
2. 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.
3. shifu-plugin-*: update pom.xml and separate dependency