Added specs for #cache
#update_cache -> #update_index

Notifies with updated package sources.
#update_index notifies sources that were/not updated
No more @cache_is_dirty
Updated docs for observer updates
Fixed and added spec for #update_index
Don't need #update_cache! anymore
#packages -> #installed_packages
Added spec for #installed_packages
Added #upgrade_packages
"packages" -> "package"
Removing #search for now
Don't need @cache stuff anymore