Added pdd (post-mortem debugging) method to class Pdb.

Moved class Cmd out to module cmd.py.
Rewrote implementation of stack trace to incorporate traceback objects.

Check for closed objects in getattr (not perfect, but better).

Added expanduser() and normcase() and isabs() and isfile().

Simple interface "pdb.run('<statement>')" added.

Rewrote has_magic using a regular expression match.

Split off file cache to module 'linecache'.