Support indexing in DenseMatrices in PySpark

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #5232 from MechCoder/SPARK-6576 and squashes the following commits:

a735078 [MechCoder] Change bounds
a062025 [MechCoder] Matrices are stored in column order
7917bc1 [MechCoder] [SPARK-6576] DenseMatrix in PySpark should support indexing