This enumerates all properties and their values and can be very expensive.

Many more to deal with; `make spelling` for the list

This allows FS subclasses to provide more efficient implementations, and have
them transparently used through various wrapper classes.

The main class is now CacheFSMixin, a mixin class that can add caching to
any FS implementation.  It operates explicitly on a PathMap of recent 
info dicts, making it more efficient and more robust than the old CacheFS.

The CacheFS wrapper class is still there, but it's a trivial class that
just mixes CacheFSMixin into the base WrapFS class.

TahoeFS has also been modified to use the mixin architecture.

This will really only benefit if the server sends the results incrementally
(e.g. via chunked-encoding) but it can't hurt in the general case anyway.

This module allows you to import python modules from the files in an FS.
So you can now import modules directly over HTTP, SFTP, or even from a
zipfile ;-)