have the track process return (stdout, stderr) tuple

added to Hive module as well.

This allows you to capture the output of a hive query directly, eg:

```python
class TestTask(luigi.hive.HiveQueryTask):
  def query(self):
    return """ select count(1) from test """
  def run(self):
    out = self.output().open('w')
    (stdout, stderr) = self.job_runner().run_job(self)
    out.write(stdout)
    out.close()

  def output(self):
    return luigi.LocalTarget("test_data/hive_output.log")
```