Docs (NO MERGE NEEDED)

@twobraids (cc @selenamarie @rhelmer )

This is my start. I'm not done yet but I've got a decent start on it. 

Note that I pushed this branch into mozilla/crontabber so you can commit into it. 

You can either work on it by going to https://github.com/mozilla/crontabber/tree/docs/docs and clicking Edit in GitHub on the .rst files. 
Or you can download the branch and run:

```
pip install sphinx
cd docs
make html
```

Feel free to edit away as much as you like.

**In particular** I feel like I didn't get the part of [Postgres specific apps](https://github.com/mozilla/crontabber/blob/docs/docs/user/moreadvancedapps.rst#postgres-specific-apps) right and that's really where I need your help lars. 

Pieces still to work on:

- [ ] Nagios
- [ ] How to start crontabber from bash
- [ ] Using advanced settings (`required_config`) in apps and how that relates to `crontabber.ini`
- [ ] Backfill jobs
- [ ] Contributor docs and how to run tests
- [ ] Configure raven
- [ ] How to use `JobStateDatabase` from "outside" crontabber
- [ ] How to configtest 
- [ ] How to reset jobs