initial implementation of nfs3 COMMIT

added a commit op to VirtualFileSystem interface.
for chimera commit is a nop since it should never be called (all IO is done FILE_SYNC)
for PseudoFS commit is a pass-through (nfs3 rfc does not include access/permission check for commit)
for VfsCache commit is also pass-through (same as write)
Merge pull request #14 from radai-rosenblatt/master

initial implementation of nfs3 COMMIT