Enable use of HTTP cache headers in curl plugin

Enable use of HTTP cache headers in curl plugin

This will allow us to get 304 responses back from remote URLs that we are
grabbing using the curl, weather, and rss plugins. The first time we fetch a
resource, we will always get the full content, but from there on out we will
store any provided 'Etag' or 'Last-Modified' header, and submit these on the
next request. If we get a 304 response back, we won't have to do any work at
all.

This benefits both us (bandwidth and parsing savings) and remote URLs (we
actually make an attempt to not retrieve the same resource over and over
again).

Signed-off-by: Brenden Matthews <brenden@diddyinc.com>