Implementation of file_exists new uses MultiStorageFieldFile.name rather than filer.File.path

Apparently many storage backends don't implement path. Looking at the docstring in django.core.files.Storage.path, this makes sense:
"Storage systems that can't be accessed using open() should *not* implement this method"
MultiStorageFieldFile.name works correctly with multiple storage backends
Merge remote-tracking branch 'origin/master_pbs' into master_pbs