H2OVector: rework

- H2OVector is now an abstract class in front of H2OColumn and H2ORow
- Implement minValue() and maxValue() methods
- H2OMatrix uses H2OColumn directly (without H2OVector abstraction)

- Indentation fixes
H2ORow: put it to use

- Implement viewRow() in H2OMatrix, return H2ORow of that row
- Move all H2O-ness into KMeansDriver.
- KMeans source is now "pure" Mahout'ish (zero H2O-ness)