Forward etag and cache-control (and set proper mime) for /issues/lables.

Issue #239. and Issue #241.
proxy_request should return the Response object, like GitHub-Flask does.
Forward caching headers for /api/issues/number.

Issue #239 & #241.
Get rid of ensure_xhr decorator (mostly useless, IMO).
Add caching headers to the homepage "browse" endpoints.

Issues #239 & #241.
Fix posting label changes.
Return the correct mimetype and add cache headers for the /comments endpoints. Issue #239 #241.
Return proper mimetype and response status when an issue is
opened/closed. #241.
Remove obsolete test.
Test the new "browse issues" sections on the homepage.