[COOK-2762] add CodePlex.download_url helper

Downloading files from CodePlex requires a cookie and a Request 
Verification Token. This helper makes it easy to capture the cookie and 
token with an initial GET request followed by a post which returns JSON 
with a final download URL for the file.
[COOK-2762] resolve download urls using CodePlex.download_url
Merge pull request #1 from opscode-cookbooks/COOK-2762

COOK-2762