Add repository config and mandatory timezone

This is hopefully (mostly) uncontroversial.

 My **big** question is what format we keep the config in. Currently extending `TextStorage` and expecting `key=value` properties by using dodgy `split()`. Arguably could use `Properties.load()` or something else. A little worried about extensibility, and wondering whether an ini-style file wouldn't be better. Happy to use Ermine and do this properly.
Did we want to wrap/hide this value in our own `IvoryDateTimeZone` to avoid depending on joda in more places?