Composer support for sites/all/vendor and sites/$sitename/vendor

Here is a patch. Should do the job. But needs some testing!

One question would be: Is there any reason to not do this by default? E.g. should we make this optional, so the user needs to enable the feature? I don't really see why, I don't think there could be anything other than the composer stuff in sites/*/vendor/composer/autoload_*.php.

And btw, why not use autoload.php created by composer?
(See also #1433256-20: Proposal: Composer as a Library Manager ff)

Because:
- Performance (1): xautoload has an APC cache, that will also be used for composer. More funky cache mechanics might be added in the future.
- Performance (2): The fewer separate class loaders we register with spl_autoload_register(), the better. (and unlike classloader, xautoload does not get slower with more namespaces)
- Flexibility: It allows to register more than one composer dir, without any duplicate classes being defined.

Caveat:
- Do we know for sure that composer/autoload_namespaces.php will always be provided by composer?
- Do we know for sure that sites/*/vendor will be the definitive place for composer stuff in D7 ?

I think this should be discussed on the Composer manager queue.
#1943536: Integrate with contributed class loader modules