[SPARK-7219] [MLLIB] Output feature attributes in HashingTF

This PR updates `HashingTF` to output ML attributes that tell the number of features in the output column. We need to expand `UnaryTransformer` to support output metadata. A `df outputMetadata: Metadata` is not sufficient because the metadata may also depends on the input data. Though this is not true for `HashingTF`, I think it is reasonable to update `UnaryTransformer` in a separate PR. `checkParams` is added to verify common requirements for params. I will send a separate PR to use it in other test suites. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #6308 from mengxr/SPARK-7219 and squashes the following commits:

9bd2922 [Xiangrui Meng] address comments
e82a68a [Xiangrui Meng] remove sqlContext from test suite
995535b [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-7219
2194703 [Xiangrui Meng] add test for attributes
178ae23 [Xiangrui Meng] update HashingTF with tests
91a6106 [Xiangrui Meng] WIP