Refactor bisecting k-means

[mengxr]

@yu-iskw This PR contains the refactoring code based on our offline discussion.

[yu-iskw]

LGTM, merging this. Thank you for this!

I think we still have a few points to discuss. As you suggested, these should be supported in Spark 1.7. One important thing is that we should design the fist commit to leave room for improvement. We can modify that without changing the top-level API.

• Support other distance metrics
  • ex) Some NLP guys requested me to support cosine distance
• Support other costs to evaluate the clusters, such as entropy, instead of average costs
• Export the dendrogram
  • ex) adjacency list, linkage matrix
• Calculate dendrogram distance between two leaf clusters
• Find the best k by cutting a cluster tree without recomputation