minor code clean-up in dirnode.py

minor code clean-up in dirnode.py
Impose micro-POLA by passing only the writekey instead of the whole node object to {{{_encrypt_rw_uri()}}}. Remove DummyImmutableFileNode in nodemaker.py, which is obviated by this. Add micro-optimization by precomputing the netstring of the empty string and branching on whether the writekey is present or not outside of {{{_encrypt_rw_uri()}}}. Add doc about writekey to docstring.
fixes #967