[SPARK-7753] [MLLIB] Update KernelDensity API

Update `KernelDensity` API to make it extensible to different kernels in the future. `bandwidth` is used instead of `standardDeviation`. The static `kernelDensity` method is removed from `Statistics`. The implementation is updated using BLAS, while the algorithm remains the same. sryza srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #6279 from mengxr/SPARK-7753 and squashes the following commits:

4cdfadc [Xiangrui Meng] add example code in the doc
767fd5a [Xiangrui Meng] update KernelDensity API