modification to the unencrypted cookie to use a clearer api

improved the signing to use a derived key based on a random salt, and
upgraded the hash from sha1 to sha512. Finally the entire result is b64
instead of just the payload.
introduce SignedCookieSessionFactory

- Break apart UnencryptedCookieSessionFactoryConfig into a
  BaseCookieSessionFactory.

- Add support for reissue_time in the base. Set the unencrypted class to
  use reissue_time=0 for bw-compat.

- Add SignedCookieSessionFactory which wraps the base in a serializer
  that uses signing via a sha512+hmac with a secret derived using an
  8-byte random salt.
fix py3
make these tests pass on python 3.2+
Revert "make these tests pass on python 3.2+"

This reverts commit cd218d2934c87260bbb10620e3b419b275fe6244.
Merge branch 'master' of github.com:Pylons/pyramid
update session to use a static salt and separate serialize funcs
add deprecation for old cookie factory
fix tests on python 3
updated changelog
update the docs
reference github issues
Merge branch 'master' into feature.signed-cookie-session
use zope.deprecation for warning about the UnencryptedCookieSessionFactoryConfig deprecation (it will happen at import time, rather than usage time, which is good for tests); add a few sphinx directives for deprecated and versionadded
add admonishment against secret sharing
mon
remove redundant "see"
moar typos
remove unnecessary length check, slices are magic
rewording about deprecation and cookie compatibility