Asynchronous map downloading.

This replaces the janky and crashy map download support with a much smoother implementation:
![async](https://f.cloud.github.com/assets/167819/2444122/731fba66-ae4a-11e3-8a30-be0664e63c90.png)

Admins attempting to force start a game are now prompted with a confirmation dialog pointing out that this is a rather rude thing to do, and also that players will be kicked if anybody is missing the map:
![kick](https://f.cloud.github.com/assets/167819/2444140/e736f3a6-ae4a-11e3-9a4c-6a9761c54ab9.png)

This also uses the resource website to fill in unknown map details in the server browser (note the RA and C&C maps in the D2K chooser):
![browser](https://f.cloud.github.com/assets/167819/2444129/8f53f224-ae4a-11e3-84b0-bc5eb007c07f.png)