Option to _not_ cache aliased paths

When working with i18n we want to avoid caching the original path. For example

1) French user navigates to example.com/fr
2) Boost caches /fr.html and /index.html
3) Other spanish user navigates to example.com
4) Apache serves up the Boost cached page in French

The solution is to not cache the original/system/aliased path. However there is a slight performance advantage for non-i18n sites to cache both. So this patch adds a new advanced setting to control whether Boost caches the system path.