do not reuse feature vector
More modularization and refactoring:

- Move ssh and shell commands to their own file.
- Use classes to represent test suites; factor out some common code.
- Add Cluster class for encapsulating interaction with a Spark cluster.
Merge pull request #5 from mengxr/mllib-fix

do not reuse feature vector
Merge remote-tracking branch 'db/master' into refactoringMerge
"Fixed merge conflicts"
Conflicts:
	lib/sparkperf/main.py