Fixed intermittent failures by making MVCCMappingStorage hold a per-connection snapshot of the database.

This changed the schema, so we have to use the schema
differently. Presumably the schema is private.

Added an option to disallow cross-database references.

debugging easier.

Include extra data in InvalidObjectReference exceptions to make
debugging easier.

Added an option to disallow cross-database references.

did not necessarily increase for every transaction.  Fixed and tested.

object can now *also* be used as an object with attributes mapped to
the root-object keys.

support.

  Python (2.5 and later) with statement::

db = ZODB.DB(...)
     with db.transaction() as conn:
          # ... do stuff with conn

This uses a private transaction manager for the connection.
  If control exists the block without an error, the transaction is
  committed, otherwise, it is aborted.

- Connections now have a public ``opened`` attribute that is true when
  the connection is open, and false otherwise.  When true, it is the
  seconds since the epoch (time.time()) when the connection was
  opened. This is a renaming of the previous ``_opened`` private
  variable.

output.

convenient way to open a connection to a database.  They open a
database and return a connection to it. When the connection is
closed, the database is closed as well.

messages.

the time zone definition, only worked on Unix.