oc-1875 : running OwnCloud on an alias (or not) with Nginx [the real one]

This Pull Request is a similar to https://github.com/owncloud/core/pull/22 (the #22 could be deleted).
This is another attempt to do things properly.
I hope this Pull Request is clean and done properly :

* 1 commit
* dedicated branch
* rebased and up to date with owncloud/core

For my first "Pull Request" attempt I wrote something like this : 

> Hi, this is the code that I described in my recent bug report:
> http://bugs.owncloud.org/thebuggenie/owncloud/issues/oc-1875
> 
> Purpose is to use Owncloud as an alias with Nginx¹. The code calculate OC::$SUBURI and OC::$WEBROOT using $_SERVER

¹ Nginx or other HTTPd server