Use rsvg to generate pngs from vector files

[willgearty]

Sometimes there isn't a png of the desired size available on phylopic (e.g., #46). We now catch this and throw and warning and fall back to the largest available png (#47), but sometimes only a very small png is available, which might not be desirable. Now, as we've discussed, we don't really want to encourage the use of pngs, but it should be fairly straightforward to just generate the pngs ourself from phylopic's vector.svg image using rsvg_png and readPNG. This way users could theoretically specify any desired png size and we would be able to make it for them (although perhaps we could still download the existing png if it exists for the desired size to be faster).