Implement support for custom http cache & class loader

Closes #53

Based on work by @patrickallaert, but adapted for the comments in #53 to
support custom class loader, and also added in the new apache/nginx vhost doc.

Oh, and removed Apc stuff as this can be archived with the new variables instead now.

ping @lolautruche @dpobel @bdunogier 
Technically, the NS doesn't have to be absolute, since it is used from the root NS.