display addr:housenumber at z17+

Small fix: 

As of now, 
![address-labels-avant](https://f.cloud.github.com/assets/955351/944152/790a6b80-029a-11e3-9a54-0f00ffe0f15d.png)
Address House numbers were being displayed without any styling at zoom 15, as shown: 

With this fix, Address numbers only display at zoom 17 or higher (same zoom level as HDM-CartoCSS). 
I realize that we should only match the styling of the HDM-CartoCSS where it's appropriate and in this case, it is appropriate. 

After, with this pull request:
![address-numbers-apres](https://f.cloud.github.com/assets/955351/944165/29f909b0-029b-11e3-8f6a-ffcdce065772.png)