autoloader: PSR-0 compliance, find generic classes via PSR-0 relative paths

Firstly, this makes the autoloader more PSR-0 compliant: it allows
underscores in namespaces, as the spec requires (based on the PSR-0
spec's reference loader, all errors my own). Secondly, instead of
defining prefixes for specific classes and unconditionally including
them in the result of findClass, it makes findClass simply return a
PSR-0 style relative path for non-OC classes, and adds the appropriate
top level directories for modules that are not already PSR-0 compliant
from 3rdparty/ to the include_path in base.php. This allows for the
possibility that the libraries will not be found in the OwnCloud
3rdparty tree and instead a systemwide copy should be loaded (as
downstream distributions might like to do).