Extensions homogenization, cleaned up.

Rebased and cleaned up version of extension homogenisation. I will give some comments that should be fixed before merge now.
I think, this explanation is best:

> The optional argument can load OcsigenServer's extensions BY DEFAULT according to the configuration file. Otherwise, it waits a server configuration (like [this line](https://github.com/ocsigen/ocsigenserver/pull/52/files#diff-98999196fb134cd7914779583b67824eR188)). In the latter case, you must load extensions by hand (using the `init` function) before the application of this function.