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 
We could remote the `$loader = ` since we don't use the variable, couldn't we ?