add CookbookResource#download_file for downloading single files from a cookbook
add Connection#stream function for streaming responses into files

refactor Chef Auth middleware and test it
implement test for CookbookResource#download_file
write to tempfile before copying contents to destination while streaming

handle SSL verification disabling in streaming requests
remove focus: true from an rspec test
Merge pull request #57 from reset/downloading-cookbook-files

Add ability to download single files from a CookbookResource