add Gist#rawify

for anonymous users, the url returns from gist api is something like
https://gist.github.com/7216441 , and it would redirect to
https://gist.github.com/anonymous/7216441, it's not easy to get
the raw file url by scripting.

this commit make the hack to handle the HTTPRedirection, and concate
the result url with /raw to generate the raw url.
add -r option, and allow use combination with -s
add spec for Gist#rawify
modify -r to -R

gist may support recursively gist all files in a directory by `-r`
add comment for rawify
Merge branch 'add-raw-option'