Update hdfs.py

Minimum viable change to allow setting an override to HDFS tmp dir in luigi.
Don't try to mail errors when missing config.
Barebones sphinx setup.

The index page isn't very exciting, yet.
Merge pull request #271 from jcrobak/dont-email-none

Don't try to mail errors when missing config.
Fix the top_artists example
Merge pull request #273 from rantav/top-artists-fix

Fix the top_artists example
Update documentation for luigi.task.
Update documentation for luigi.parameter.
Add sphinx documentation for the api.
Merge pull request #272 from jcrobak/sphinx

Sphinx docs
Merge pull request #277 from cannonpr/master

Update hdfs.py