Option to enforce storage limits at all times

Uses storage enforcement ideas from #793 but can now only dump stuff from stores when you get the stores overfull screen, not transfers or craft anymore, because the UI overhaul is dropped, with the only UI change to show the space used on the purchase and sell screens if the option is enabled.

![screen062](https://cloud.githubusercontent.com/assets/1824834/2845122/0d8d26d2-d093-11e3-81c6-c391d99dadd2.png)
![screen063](https://cloud.githubusercontent.com/assets/1824834/2845126/12e1d312-d093-11e3-8e04-417792311672.png)