Refactoring: roloid vs useroid

Add comments to all columns of *job tables/views
Wrote crontab parser.
Added testcases for crontab.
Added testcases (random crontab entries).
Altered the cronparser to mirror the behaviour of Vixie Cron (weekdays and doyu of month behaviour).
Get all the jobs for a given timestamp: slow, because not using the parse_timestapms index, but doing a seq scan
Finding jobs is now using the index.
For debugging purposes the function to get the jobs at a certain time
is converted from sql into pgsql. We can now add some RAISE statements etc if needed.
Parsing the timestamps now works.

The schedule column currently is text, we will probably need to change this into a DOMAIN for clarity.
Changed schedule datatype into the schedule domain.
Moved the check constraints on the table to the domain.

Added comments on many objects.
Merge pull request #3 from zalando/data_model

Data model: comments, domains, hardening