unmount now closes the filesystem after unmounting it.
Use 'r' in mode since mode might be 'rb'.
Rename needs to handle renaming archives (don't redirect through an ArchiveFS).

If the wrapped_fs reference is not removed, the SubFS will create a cycle with it, preventing both from being garbage collected.

Prevent MemoryFS.removedir(..., recursive=True) removing non-empty parent directories, and fixed up corresponding unit test.
Fixes Issue 137

Ensure that pyfs file systems are closed when use with ftp and sftp daemons.

Synchronize the close() methods.
Ensure the close() methods close and del referenced file systems.

isfile(). It did not properly handle the case of a path within a mountable archive.

they behave properly when dealing with archive files (instead of archive contents).

A couple of these changes were also driven by the need to mix ArchiveFS mount points
with mount points hosting other file systems (like remote file systems).

archives themselves to be handled. Without these changes, an archive would be automatically
mounted, once mounted, operations on the archive file itself would fail. 

Also added a max_size parameter to ArchiveMountFS, to avoid mounting really large archives.

Forgot to add BackReferenceError to __all__ list. This caused problems in expose/sftp.py (BackReferenceError not defined).

longer has the problems we were encountering before. Also required a quick
fix to SFTPFS.removeddir(..., recursive=True)