Use cache_control decorator instead of cache_page

I used cache_page originally as it sets all the relevant HTTP headers
("Cache-Control" and "Expires") to ask browsers/proxys to cache the
generated image to reduce load.

What I didn't know at the time is that cache_page also inserts the
generated images into the cache backend. Sites with many images being
served thorugh the "Frontend URLs" mechanism could end up with their
cache servers being overloaded.

This pull request replaces the use of cache_page with cache_control
which doesn't insert the image into the cache.

Also, cache_control only sets the "Cache-Control" HTTP header (and not
"Expires"). As "Cache-Control" is defined in the HTTP 1.1 spec, I think
it's safe to assume that the vast majority of browsers and proxys
support it.
PEP8 fix
Merge pull request #652 from kaedroho/image-frontend-cache-fix

Use cache_control decorator instead of cache_page