Add encryption to the object cache

Just doing some work want was wanting to gauge the likely hood of a patch being accepted which changes the object cache so that it will optionally encrypt the contents on disk.

I was thinking of changing the following functions like so

function ctools_object_cache_set($obj, $name, $cache, $enc_key = NULL)
function ctools_object_cache_get($obj, $name, $skip_cache = FALSE, $enc_key)

So when it is writing the object to the database it will encrypt and decrypt the data blob.

I was actually thinking that I could use the form_build_id as the encryption key which I think should be pretty safe since it is a completely random key and is generally only stored in the current form.

I was thinking of use mcrypt_encrypt() and mcrypt_decrypt() to do these and add in a check so that it would only do it if the mcrypt extension is loaded.