Evaluation metrics and stream generators

Memory and runtime evaluation metrics added, gradual stream generator
implemented. All stream generation classes were moved to the
datastream.stream package.
Merge remote-tracking branch 'origin/master'

Conflicts:
	moa-pom/moa/src/test/java/Experiment.java
	moa-pom/moa/src/test/java/TestSuite.java
	moa-pom/moa/src/test/java/datastream/streams/StaggerImbalanced.java