Changed to use has_key rather than get to work with older mapping objects.

Added important note in constructor.

Changed to use keys() rather than items to iterate over
the index.

  - Provides a relatively simple example for building full storages,

  - Is useful for giving demos because it is non-persistent.

Took out incomplete code for allowing a base base storage.  Base storages
are trickier than they first seem.

in transaction writing *just* in case a system crashes during the last
step of commiting a transaction.

all transaction aware storage managers recognize subtransactions.

(in another connection).

pool to go south when there were more app threads than connections.

during an active transaction.

Added subtransaction support.

connection with the stored data.  This is handy when multiple
threads or processes are accessing and updating data simultaneously
(e.g. When using the monitor client to poke at a running Zope process).

This at least makes errors visible during debugging,
but I need to figure out a better way to handle these.