Add support for Etags

Well, I took a stab at adding etags and it came out ok. I wasn't sure where to put the test, so it ended up in `spec/marvelite/api/client_spec.rb` - if there's a better place, I'd be happy to move it.

I'm also open to any other feedback you might have. I did update the README, have a look and let me know what you think.

Thanks!
Jon