Factor out HTTPAdapter.proxy_manager_for for #2048
update AUTHORS.rst
improve `proxy_manager_for` re-usability by subclass
allow pool_kwargs to be specified in init_poolmanager
Check for basestring, not just str

Fixes #2071
Remove unnecessary check for builtin_str
Ensure that we open files in binary mode.
Merge pull request #2074 from Lukasa/filedoc

Ensure that we open files in binary mode in the docs.
Merge pull request #2072 from sigmavirus24/fix-urlencoded-bytes

Check for basestring, not just str
Allow copying of PreparedRequests without headers/cookies
Merge pull request #2078 from dgouldin/2077

Allow copying of PreparedRequests without headers/cookies
v2.3.1 changelog
Update trivial mentions to RFC 2616.
Force basic auth strings to native string type
Avoid unnecessary encode/decode cycles.
indentation change for code review
Merge pull request #2087 from Lukasa/2616

Update trivial mentions to RFC 2616.
Merge pull request #2076 from Lukasa/basic

Basic Auth handler sets unicode header value on Python 2
various pep-8 cleanups; remove unsused imports; remove unused variables
Update with style considerations by @kennethreitz