Move the HTTP Proxy code to a single function inside utils.js

Right now, the line `var proxy = process.env.proxy || process.env.HTTP_PROXY || null` is repeated across the module. Also, the case is inconsistent in a lot of places. I can move all of the occurences into utils.js and call a function from there at the top of each file (such `upload.js` and refer to that for the rest of the file)

I will work on this, if you would be interested in this patch.
