Accept `--runner Luigi`, not `--runner LuigiRunner`

The pythonic way is that there should only be one way to do things. This
will lead to less conflicts and that people are making their
configuration files consistent.
Mention how to run it in README [fixes #8]
Update conf/scalegrease.json
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 #7 from Tarrasch/less-logic-more-convention

Accept `--runner Luigi`, not `--runner LuigiRunner`
Merge pull request #9 from Tarrasch/continious-output

Log LuigiRunner subprocess output to logger
Fix build dependencies