Add detailed smarty cache documentation to config

Instead of just making it availble document the smarty cache feature.

It might work for users, but it's advised to rely on the memcache
instead.

Fixes #309
Adding cache lifetime option to smarty config

* Renamed configuration array to `smarty` => `cache`
* Added `smarty` => `cache_lifetime` to expire cache files properly

This should be safe to use, be aware that each page request is cached!
That includes any POST/GET calls to the site.

It does help in speeding up the site, up to 100% on some requests. For a
high traffic site it probably makes sense to enable this option with a
low cache lifetime to ensure most recent data.

Addresses #309
adjust smarty configurations