title: get bounding box from ggmap object
body: <p>I'm using ggmap library in R. I'm trying to download a rectangular map with it, but I know it'll give me a square. I only need the bounding box of the returned square.</p>
  
  <pre><code>library(ggmap)
  map &lt;- get_map(c(-65.7,-3.1,-64.4,-2.3),maptype="satellite",filename="map.png")
  str(map)
  
   chr [1:1280, 1:1280] "#294829" "#294829" "#2D512D" "#264425" ...
   - attr(*, "class")= chr [1:2] "ggmap" "raster"
   - attr(*, "bb")='data.frame':  1 obs. of  4 variables:
    ..$ ll.lat: num -3.14
    ..$ ll.lon: num -65.5
    ..$ ur.lat: num -2.26
    ..$ ur.lon: num -64.6
  </code></pre>
  
  <p>Object <em>map</em> have two classes "ggmap" and "raster". I can't use @ or $ in it. How then can I access the ll.lat and other attributes from "bb" sub-object?</p>