Fix Worker for use with DATABASE_URL

Closes #225
Add tests for Worker initialization
Merge pull request #226 from cschramm/master

Fix Worker for use with DATABASE_URL
removing date and changing homepage in gemspec
bump version
change ref to versions in README to fit reality
fix repo path in README
Support for scheduling jobs in the future
Changed scheduling to use an offset based approach
Documentation for #enqueue_at/#enqueue_in
Moved scheduling timestamp into its own column and created separate migrations

Since the release of 3.0.0 the created_at column is "out in the wild"
and probably being used to keep track of when a job was created. It
should not be used for something semantically completely different like
scheduling therefore I introduced a new column for that aspect.