Version 2.3.0

There's been over a month since the last release. This new release includes a
bunch of new features. What I like the most is that they all come with full and
proper documentation!

luigi:

• Fixed lockfile bugfix introduced in 2.2.0 #1809 Thanks @nugend
• Retcodes improved #1771 Thanks @fabriziodemaria
• Added NumericalParameter
  and ChoiceParameter Thanks @brcopeland
• Per-task configurable retry policies! #1791 Thanks @javrasya
• Luigi can now batch tasks! #1784 Thanks @daveFNbuck

luigi.scheduler:

• We now have a new definition of the UPSTREAM statuses. New intuition is that
  UPSTREAM_FAILED means that that task cannot run because all downstream
  tasks have failed or worse (like being disabled). As an effect there will be
  much much fewer tasks considered to have an upstream status. #1789

There have been a lot of other features, cleanups and bugfixes! See all commits
here.