Log LuigiRunner subprocess output to logger

The output will now look something like this, note how we are not only
printing the output but also prefix it with the logger information like
timestamp and stuff. It looks a bit verbose but it makes it clear what
says what.

    $ ./bin/greaserun --config-file conf/scalegrease.json  --verbose --runner luigi README.md
    INFO:root:Reading configuration from conf/scalegrease.json
    DEBUG:root:Configuration read:
    {u'HadoopRunner': {u'command': [u'hadoop', u'jar']}, u'_note': u'Avoid environment variables in production. They are useful for testing, however.', u'launch': {u'crontab_repository_host': u'$SCALEGREASE_REPO_HOST', u'crontab_repository_dir': u'$SCALEGREASE_REPO_DIR'}, u'LuigiRunner': {u'command': u'spluigi'}, u'ShellRunner': {}, u'runners': [u'scalegrease.luigi.LuigiRunner', u'scalegrease.hadoop.HadoopRunner', u'scalegrease.runner.ShellRunner']}
    INFO:subprocess./home/arash/spotify/repos/scalegrease/slow_printer.py:0
    INFO:subprocess./home/arash/spotify/repos/scalegrease/slow_printer.py:1
    INFO:subprocess./home/arash/spotify/repos/scalegrease/slow_printer.py:2
    INFO:subprocess./home/arash/spotify/repos/scalegrease/slow_printer.py:3
    INFO:subprocess./home/arash/spotify/repos/scalegrease/slow_printer.py:4

https://jira.spotify.net/browse/DATAEX-90
Merge pull request #9 from Tarrasch/continious-output

Log LuigiRunner subprocess output to logger