Request raw data for gists that are truncated

Gist files greater than 1 MB are truncated in the gist API.
https://developer.github.com/changes/2014-05-06-gist-api-now-truncating-large-files/.
Look for the truncated flag and request the full contents from the
raw_url if it's set. 

Addresses #364.

A few days ago this fix didn't work - gist raw files are served from gist.githubusercontent.com, which was not CORS enabled. But after an email, @github support just set `Access-Control-Allow-Origin: *`. So this works now.