Create ggmapFileDrawer if not found.

Simple addition of a few lines to address #35.

Hi dkahle.  I am so impressed with the ggmap package.  I have all sorts of uses for it.

I had some issues with the CRAN version and png files not being found, so I `devtools::github_installed` your github version.

In that version I had some issues with the ggmapFileDrawer not being found.  That same issue was found in #35, so I added a few lines to resolve that.

Cheers,

eric