Change map container background to light gray.

This prevents the page looking weird and incomplete when it loads. The map switches to gray anyway once it starts loading the tiles, so this just accelerates this.

Screenshot of the “before” situation with white background:
![screen shot 2013-11-11 at 11 48 50 am](https://f.cloud.github.com/assets/2061609/1515930/26194e0a-4b0b-11e3-9faf-19e5f8ffc4f5.png)