refactory to reduce responsibility of HBaseRelation

In addition to PR 35 (https://github.com/Huawei-Spark/spark/pull/35), this PR reduce the responsibility of HBasesRelation which I think it is good for maintaining compatibility in the future.
Since only HBaseSQLReaderRDD is using buildXXX and getPrunedPartition functions, so I move them into HBaseSQLReaderRDD.

I have verified the modified code with simple test in the CLI