Several changes to make this a more generally useful module.

Most important: a semantic change.  When a lock is acquired, an
additional float value is written to the lock file.  This is the
lock's expected lifetime, as an instant some point in the future
(i.e. time.time() + lifetime).  This allows processes to give a clue
to other claimants as to how long the lock holder intends to keep the
lock.  This is necessary because the same resource may need to be
locked by short lived and long lived processes (e.g. the archiver).
Without this, the short lived process has no idea when the lock owner
should have given up the lock and could steal it out from under it.

It is possible that a process could continually refresh a lock (see
below) in an infloop, thus causing all other claimants to back off
forever.  This is (I believe) much less likely than that a process
crashes, leaving a lock turd around.

Rename this file to LockFile.py (retain flock.py for CVS
archival purposes, but soon all references to this module will be
changed to use LockFile instead of flock).  The main class in this
module is also named LockFile.

All the exceptions have been changed to class-based.  LockError is the
base exception class.  AlreadyCalledLockError is changed to
AlreadyLockedError but NotLockedError and TimeOutError are not
changed.

New public methods set_lifetime() and refresh().  The former sets the
lifetime interval so that the same lock object can be reused with a
different lock lifetime.  The latter refreshes the lock lifetime for
an already held lock.  A process can use this if it suddenly realizes
it needs more time to complete its work.

"hung_timeout" is renamed to lifetime, because of the changed semantics

All private attributes have been __renamed

Docstrings everywhere!

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