Support plain .tar cookbooks as well as .tar.gz cookbooks.
This is needed to install afw cookbook http://community.opscode.com/cookbooks/afw
Check gzip or tar file signatures in Berkshelf::CommunityREST.unpack.
Fix signature check of gzip files.
Use Array#pack to create a gzip file signature.
Fix 'unpacks the tar' spec.
Merge pull request #452 from hnakamur/support_plain_tar_cookbook

Support plain .tar cookbooks as well as .tar.gz cookbooks.