CLean up Printer __init__

Removed mutable types from the default arguments for the Printer
__init__. Using mutable types in default arguments in Python is messy
because the default value is evaluated only once, which means changes to
those values will be shared across successive calls. This had no
negative effect in this case because only one Printer instance was
created each time Redeem was run, but it could cause problems down the
road if unchanged.

See
https://docs.python.org/2/tutorial/controlflow.html#default-argument-values