Use constant time hash comparisons for improved security.

See e.g. http://codahale.com/a-lesson-in-timing-attacks/

Mega thanks to Alex Gaynor for the original patch.
Changelog re hash comparison bugfix
Merge branch '1.10' into 1.11

Conflicts:
	sites/www/changelog.rst
Merge branch '1.11' into 1.12

Conflicts:
	paramiko/hostkeys.py
	sites/www/changelog.rst