Configurable startup directory with a default that make sense :smile:

I implemented a startup directory adapter for the reference field of the shopitemblock.

Reason:
If you are using ftw.subsite Subsites (NavigationRoot) and the shop is not placed in the subsite, but generally below the plone root. In this case the user was unable to reach the shop. The Widget only shows the tree within the "next" NavigationRoot. 

The implemented default behaviour is now to set the startup directory to the IShopRoot marked content. 
Since the adapter adapts context and request it's easily possible to register a specific one for other use cases. 

![screen shot 2013-12-04 at 11 37 34 am](https://f.cloud.github.com/assets/437933/1672392/f6c8eebc-5cd0-11e3-861f-607fbd1e2554.png)

@lukasgraf