Stop boundary linestrings from relations being used

I have yet to do full tests, but this fixes #344 at the Canada/US border, which is one of the worse borders for overlapping edges of admin areas.

At z14

New
![image](https://f.cloud.github.com/assets/1190866/2228523/0899388c-9add-11e3-8104-02a71b1ee5b4.png)

Old
![image](https://f.cloud.github.com/assets/1190866/2228535/670b989c-9add-11e3-9e5d-72536876b082.png)

Names are still an issue on z15+ as they're coming from the fallbacks, but one problem at a time