Perform file locking on win32

The functions _insert_bytes_no_mmap and _delete_bytes_no_mmap call
mutagen.util.lock, which in turn uses the fnctl module which isn't
available on Windows.
Merge pull request #178 from mwiencek/win32-locking

PICARD-437: Picard can have problems saving files to network shares on Windows 8(?) when embedding cover art