Add auto-docs for new luigi.contrib packages.
Fix luigi.s3 and luigi.webhdfs default imports

Don't fail when importing luigi.s3 or luigi.webhdfs if deps
aren't satisified. This improves the ability to generate api
docs for these modules when required packages aren't installed,
and it is in line with the pattern used in luigi.{hdfs,hive,postgres}
and other places.
Merge pull request #427 from jcrobak/doc-fixes

Doc fixes