[SPARK-5116][MLlib] Add extractor for SparseVector and DenseVector

Add extractor for SparseVector and DenseVector in MLlib to save some code while performing pattern matching on Vectors. For example, previously we may use:

     vec match {
          case dv: DenseVector =>
            val values = dv.values
            ...
          case sv: SparseVector =>
            val indices = sv.indices
            val values = sv.values
            val size = sv.size
            ...
      }

with extractor it is:

    vec match {
        case DenseVector(values) =>
          ...
        case SparseVector(size, indices, values) =>
          ...
    }

Author: Shuo Xiang <shuoxiangpub@gmail.com>

Closes #3919 from coderxiang/extractor and squashes the following commits:

359e8d5 [Shuo Xiang] merge master
ca5fc3e [Shuo Xiang] merge master
0b1e190 [Shuo Xiang] use extractor for vectors in RowMatrix.scala
e961805 [Shuo Xiang] use extractor for vectors in StandardScaler.scala
c2bbdaf [Shuo Xiang] use extractor for vectors in IDFscala
8433922 [Shuo Xiang] use extractor for vectors in NaiveBayes.scala and Normalizer.scala
d83c7ca [Shuo Xiang] use extractor for vectors in Vectors.scala
5523dad [Shuo Xiang] Add extractor for SparseVector and DenseVector