Barebones sphinx setup.

The index page isn't very exciting, yet.
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
Add missing information about the first example in README.md
Clean up example: use context manager when opening files (already done elsewhere in the file), use  rather than %s, remove trailing spaces on a number of lines, break lines at under 80 characters
Don't try to mail errors when missing config.
Fix the top_artists example
Add support for redshift.

Redshift is in some ways similar to postgres, so it inherits the postgres target and postgres copy task, and adds some required changes for redshift
Move redshift to contrib
Extract a common superclass to postgres and redshift
Add a call to init_copy before running the copy
Rename redshit's copy_attributes to copy_options
Make copy_options an abstract property
Merge branch 'master' of https://github.com/spotify/luigi