update cache_fu to allow action TTL, other stuff

hi

I've updated cache_fu to allow setting the ttl for action caching on a per action level. This is only enabled when fragment caching is turned on, since once you switch memcache fragments on you have to use memcache for actions also.

The syntax is as follows:

caches_action :index, :page => { :ttl => 5.minutes } # :index uses default ttl, like normal

Or:

caches_action :landing => { :ttl => 20.hours }, :show => { :ttl => 5.minutes }

We don't really need the nested hash, but that keeps it consistent with the rest of the cache_fu API.

This patch also has a couple other assorted things that I fixed as I went:

• fragment caching will now fallback to the ttl in config first, then it falls back to 25 minutes -- before it would fall back to 25 minutes and ignore the setting in the config.
• in the test helper, try adding vendor/rails to the load path if possible, so that tests will run against that if you are vendor'ing.
• use $LOAD_PATH, not $:, more readable
• rename a fragment cache spec that was a copy and paste error (had a duplicate spec name so probably wasn't running