Use thread-safe Singleton mixin for Digester
add CHANGELOG for PR#1808
Merge pull request #1870 from opscode/lcg/digester-singleton

Lcg/digester singleton
Switches to https://supermarket.getchef.com

This modifies knife cookbook site to use supermarket instead of
cookbooks.opscode.com. Additionally, it switches to using HTTPS instead
of HTTP.
Updates spec for supermarket
adding CHANGELOG for pr #1801
Merge pull request #1871 from opscode/lcg/supermarket-urls

Lcg/supermarket urls
Use thread-safe OpenSSL::Digest instead of Digest
add CHANGELOG for pr #1807
Merge pull request #1872 from opscode/lcg/digester

Lcg/digester