Move settings to DB

![screenshot 2014-07-17 20 31 44](https://cloud.githubusercontent.com/assets/812086/3617607/a5b428ae-0de0-11e4-9448-4bcd3900e19c.png)

This adds a configuration page to the administration section of uberZeit allowing to change "Hours of work per day" and "Days of vacation per year" with a simple UI. The current settings from the configuration file (`uberzeit.yml`) are automatically migrated to the new `settings` table.

Kudos to @luxflux, he did most of the hard work. :beers:
Use the new "sexy" validations (`validates` ...).