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

Firstly, this makes the autoloader more PRS-0 compliant: it allows
underscores in namespaces, as the spec requires. Secondly, instead of
defining prefixes for specific classes and unconditionally including
them in the result of findClass, it makes findClass simply return a
PRS-0 style relative path for non-OC classes, and adds the appropriate
top level directories for modules that are not already PRS-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).