Two more fields for issue resource.

Not a bug report this time, just a couple of suggestions to improve the API. There's two pieces of information in most comic books that I can see on the website but that I can't access through the API. I'll use this comic as an example:

1) There are a whole bunch of extra covers; I can only access the first cover with the API, but the others would be useful too.

2) There is a score/rating (4.17 in this case) for this book. This would be a useful addition to the API as well.

Just something to consider for when you guys run out of things to do. ;)

+1 for accessing the alternate covers via the API. At the moment the only way is through scraping the issue page.