Expose ETags in requests for performance

[porglezomp]

From the API page Performance Tips, on allowing you to avoid downloading redundant information.

It would be nice if I could make a request like response = client.subreddit_gallery(name, etag="a695f4e9672bf7fc7a779ac12ead684d72292506") that would set the If-None-Match header, and be able to get response.etag on responses.