Cannot download some tiles (often stamen maps)

Hi,

This is probably not related to `ggmap` itsefl but impairs its usage (and maybe you can work around it). Quite otfen, I cannot download a tile through R while using the address in a web browser works.

For example

```
qmap("new-york", zoom=13, source="stamen", maptype="watercolor")Error in download.file(urls[[k]], destfile = destfile, quiet = !messaging, :
cannot open URL 'http://tile.stamen.com/watercolor/13/2411/3078.png'
In addition: Warning message:
In download.file(urls[[k]], destfile = destfile, quiet = !messaging, :
cannot open: HTTP status was '504 Gateway Time-out'
```

while immediately after, using `http://tile.stamen.com/watercolor/13/2411/3078.png` in chrome returns the map.

However, both `curl` and `wget` also get the 504 time out.

Is there a usage limit in stamen maps?