fixes COUCHDB-2362 admin auth in dev cluster

  When running the dev cluster with dev/run and the --admin option:
  - ensure that the admin is the same on all nodes after hashing
  - otherwise cookies won't work when hopping between nodes
  (rnewson dixit on IRC)

  Includes a copy of Armin Ronacher's pbkdf2.py:
  - https://github.com/mitsuhiko/python-pbkdf2/blob/master/pbkdf2.py

  Fixes COUCHDB-2364 plaintext admin password remains visible if there are two [admin] sections