Add --raw option

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 shell scripting.

this commit make the hack to handle the HTTPRedirection, and concatenate the result url with /raw to generate the raw url.

#121