Save list of log handlers before removing them.

[psigen]

There is a bug in prpy.logger.initialize_logging where loggers are not fully removed by:
https://github.com/personalrobotics/prpy/blob/master/src/prpy/logger.py#L56

I think this is because the handlers property is doing odd stuff as it is iterated if removeHandler is called.

Here's an example of this code not working consistently:

# Supposedly remove all of the existing handlers.
base_logger = logging.getLogger()
print "Starting with these handlers: ", base_logger.handlers
for handler in base_logger.handlers:
    print "Removing: ", handler
    base_logger.removeHandler(handler)
print "Finishing with these handlers: ", base_logger.handlers

Starting with these handlers:  [<logging.StreamHandler object at 0x7fdb65567850>, <logging.StreamHandler object at 0x7fdb65567910>, <logging.StreamHandler object at 0x7fdb65567a10>, <logging.StreamHandler object at 0x7fdb65567ad0>]
# Removing:  <logging.StreamHandler object at 0x7fdb65567850>
# Removing:  <logging.StreamHandler object at 0x7fdb65567a10>
# Finishing with these handlers:  [<logging.StreamHandler object at 0x7fdb65567910>, <logging.StreamHandler object at 0x7fdb65567ad0>]