Refactor clients module.

- PEP8, pylint cleaning

- no-self methods -> functions, some of which were moved to acme or
  challenge module.

- a bit of renaming

- more Pythonic code

- Sphinx-like doc.
Simple doc fix in configurator
Clean crypto_util, add some doc
Fixed typos in crypto_util.py
Bring in @Kuba PEP-8 BranchMerge branch 'kuba-pep8'