Make loading/saving labeled data easier for pyspark users.

Also changed type check in `SparseVector` to allow numpy integers.

Author: Xiangrui Meng <meng@databricks.com>

Closes #672 from mengxr/pyspark-mllib-util and squashes the following commits:

2943fa7 [Xiangrui Meng] format docs
d61668d [Xiangrui Meng] add loadLibSVMFile and saveAsLibSVMFile to pyspark